cmake: deprecate BUILD_NO_GAP_FILL and introduce type specific gap fill

Deprecate BUILD_NO_GAP_FILL as it gives a false impression that gap
filling can be disabled in binary files.
Binary files are always gap filled due to the fact they contain no
address information. Only option for binary files is to control the gap
fill pattern. When no gap fill is enabled in binary files, then a
default pattern is used by the tool, which usually is 0x00.

Generally the pattern 0x00 leads to unnecessary flash writes, as a
flash generally contains 0xFF after an erase.
Therefore provide a gap fill pattern Kconfig setting instead, with
default value of 0xFF.

For hex-files, intel hex and s19, then gap filling is generally not
needed but in order to still support cases where gap filling is required
then this commit introduces BUILD_OUTPUT_HEX_GAP_FILL and
BUILD_OUTPUT_S19_GAP_FILL. Both settings are disabled per default.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

CMakeLists.txt

@@ -1647,10 +1647,9 @@ list(APPEND
 )
 list(APPEND post_build_byproducts ${KERNEL_MAP_NAME})
 
-if(NOT CONFIG_BUILD_NO_GAP_FILL)
 # Use ';' as separator to get proper space in resulting command.
-  set(GAP_FILL "$<TARGET_PROPERTY:bintools,elfconvert_flag_gapfill>0xff")
+set(gap_fill_prop "$<TARGET_PROPERTY:bintools,elfconvert_flag_gapfill>")
-endif()
+set(gap_fill "$<$<BOOL:${gap_fill_prop}>:${gap_fill_prop}${CONFIG_BUILD_GAP_FILL_PATTERN}>")
 
 if(CONFIG_OUTPUT_PRINT_MEMORY_USAGE)
   target_link_libraries(${logical_target_for_zephyr_elf} $<TARGET_PROPERTY:linker,memusage>)
@@ -1711,7 +1710,7 @@ if(CONFIG_BUILD_OUTPUT_HEX OR BOARD_FLASH_RUNNER STREQUAL openocd)
       post_build_commands
       COMMAND $<TARGET_PROPERTY:bintools,elfconvert_command>
               $<TARGET_PROPERTY:bintools,elfconvert_flag>
-              ${GAP_FILL}
+              $<$<BOOL:${CONFIG_BUILD_OUTPUT_HEX_GAP_FILL}>:${gap_fill}>
               $<TARGET_PROPERTY:bintools,elfconvert_flag_outtarget>ihex
               ${remove_sections_argument_list}
               $<TARGET_PROPERTY:bintools,elfconvert_flag_infile>${KERNEL_ELF_NAME}
@@ -1733,7 +1732,7 @@ if(CONFIG_BUILD_OUTPUT_BIN)
       post_build_commands
       COMMAND $<TARGET_PROPERTY:bintools,elfconvert_command>
               $<TARGET_PROPERTY:bintools,elfconvert_flag>
-              ${GAP_FILL}
+              ${gap_fill}
               $<TARGET_PROPERTY:bintools,elfconvert_flag_outtarget>binary
               ${remove_sections_argument_list}
               $<TARGET_PROPERTY:bintools,elfconvert_flag_infile>${KERNEL_ELF_NAME}
@@ -1820,7 +1819,7 @@ if(CONFIG_BUILD_OUTPUT_S19)
       post_build_commands
       COMMAND $<TARGET_PROPERTY:bintools,elfconvert_command>
               $<TARGET_PROPERTY:bintools,elfconvert_flag>
-              ${GAP_FILL}
+              $<$<BOOL:${CONFIG_BUILD_OUTPUT_S19_GAP_FILL}>:${gap_fill}>
               $<TARGET_PROPERTY:bintools,elfconvert_flag_outtarget>srec
               $<TARGET_PROPERTY:bintools,elfconvert_flag_srec_len>1
               $<TARGET_PROPERTY:bintools,elfconvert_flag_infile>${KERNEL_ELF_NAME}

Kconfig.zephyr

@@ -709,8 +709,21 @@ config CLEANUP_INTERMEDIATE_FILES
 	  from the build process. Note this breaks incremental builds, west spdx
 	  (Software Bill of Material generation), and maybe others.
 
+config BUILD_GAP_FILL_PATTERN
+	hex "Gap fill pattern"
+	default 0xFF
+	help
+	  Pattern used for gap filling of output files.
+	  This value should be set to the value of a clean flash as this can
+	  significantly reduce flash write times.
+	  This setting only defines the gap fill pattern and doesn't enable gap
+	  filling.
+	  Note: binary files are always gap filled as they contain no address
+	  information.
+
 config BUILD_NO_GAP_FILL
-	bool "Don't fill gaps in generated hex/bin/s19 files."
+	bool "Don't fill gaps in generated hex/s19 files [DEPRECATED]."
+	select DEPRECATED
 
 config BUILD_OUTPUT_HEX
 	bool "Build a binary in HEX format"
@@ -718,6 +731,12 @@ config BUILD_OUTPUT_HEX
 	  Build an Intel HEX binary zephyr/zephyr.hex in the build directory.
 	  The name of this file can be customized with CONFIG_KERNEL_BIN_NAME.
 
+config BUILD_OUTPUT_HEX_GAP_FILL
+	bool "Fill gaps in hex files"
+	depends on !BUILD_NO_GAP_FILL
+	help
+	  Fill gaps in hex based files.
+
 config BUILD_OUTPUT_BIN
 	bool "Build a binary in BIN format"
 	default y
@@ -752,6 +771,12 @@ config BUILD_OUTPUT_S19
 	  Build an S19 binary zephyr/zephyr.s19 in the build directory.
 	  The name of this file can be customized with CONFIG_KERNEL_BIN_NAME.
 
+config BUILD_OUTPUT_S19_GAP_FILL
+	bool "Fill gaps in s19 files"
+	depends on !BUILD_NO_GAP_FILL
+	help
+	  Fill gaps in s19 based files.
+
 config BUILD_OUTPUT_UF2
 	bool "Build a binary in UF2 format"
 	depends on BUILD_OUTPUT_BIN