build: make non zephyr/ include prefix configurable

Add a Kconfig to disable non prefixed includes. By setting
`CONFIG_LEGACY_INCLUDE_PATH=n` developers can disable having
`include/zephyr` in the search path. This means that includes such
as `#include <kernel.h>` will no longer work.

Internally, every header should be updated to add the `zephyr/`
prefix to the headers. Only then, will developers be able to use
this config value for their applications.

Signed-off-by: Yuval Peress <peress@google.com>

boards/x86/ehl_crb/CMakeLists.txt

@@ -5,7 +5,8 @@ set_property(GLOBAL APPEND PROPERTY extra_post_build_commands
   -c ${CMAKE_C_COMPILER}
   -o ${CMAKE_OBJCOPY}
   -i ${ZEPHYR_BASE}/include
-  -i ${ZEPHYR_BASE}/include/zephyr
+  $<$<BOOL:${CONFIG_LEGACY_INCLUDE_PATH}>:-i>
+  $<$<BOOL:${CONFIG_LEGACY_INCLUDE_PATH}>:${ZEPHYR_BASE}/include/zephyr>
   -f ${PROJECT_BINARY_DIR}/${CONFIG_KERNEL_BIN_NAME}.elf
   $<$<BOOL:${CMAKE_VERBOSE_MAKEFILE}>:--verbose>
   WORKING_DIRECTORY ${PROJECT_BINARY_DIR}

boards/x86/qemu_x86/CMakeLists.txt

@@ -3,7 +3,8 @@ set_property(GLOBAL APPEND PROPERTY extra_post_build_commands
   COMMAND ${PYTHON_EXECUTABLE} ${PROJECT_SOURCE_DIR}/arch/x86/zefi/zefi.py
   -c ${CMAKE_C_COMPILER}
   -i ${ZEPHYR_BASE}/include
-  -i ${ZEPHYR_BASE}/include/zephyr
+  $<$<BOOL:${CONFIG_LEGACY_INCLUDE_PATH}>:-i>
+  $<$<BOOL:${CONFIG_LEGACY_INCLUDE_PATH}>:${ZEPHYR_BASE}/include/zephyr>
   -o ${CMAKE_OBJCOPY}
   -f ${PROJECT_BINARY_DIR}/${CONFIG_KERNEL_BIN_NAME}.elf
   $<$<BOOL:${CMAKE_VERBOSE_MAKEFILE}>:--verbose>

boards/x86/up_squared/CMakeLists.txt

@@ -5,7 +5,8 @@ set_property(GLOBAL APPEND PROPERTY extra_post_build_commands
   -c ${CMAKE_C_COMPILER}
   -o ${CMAKE_OBJCOPY}
   -i ${ZEPHYR_BASE}/include
-  -i ${ZEPHYR_BASE}/include/zephyr
+  $<$<BOOL:${CONFIG_LEGACY_INCLUDE_PATH}>:-i>
+  $<$<BOOL:${CONFIG_LEGACY_INCLUDE_PATH}>:${ZEPHYR_BASE}/include/zephyr>
   -f ${PROJECT_BINARY_DIR}/${CONFIG_KERNEL_BIN_NAME}.elf
   $<$<BOOL:${CMAKE_VERBOSE_MAKEFILE}>:--verbose>
   WORKING_DIRECTORY ${PROJECT_BINARY_DIR}