shell: add an option for minimal default configuration

The default shell configuration has heavy flash and memory requirements,
requiring project maintainers to set many configuration options to "n"
to keep flash and memory requirements within reason.

This adds a new configuration option, CONFIG_SHELL_MINIMAL, which will
disable flash and memory heavy options by default, and allow project
maintainers to select/imply only the options they want.

On a quick test from an ARM board I'm working on, enabling this option
cut flash space requirements by ~8 KB, and memory requirements by ~1 KB.

Signed-off-by: Jack Rosenthal <jrosenth@chromium.org>

subsys/shell/Kconfig

@@ -17,6 +17,14 @@ module-str = Shell
 source "subsys/logging/Kconfig.template.log_config"
 source "subsys/shell/Kconfig.backends"
 
+config SHELL_MINIMAL
+	bool "Default config to reduce flash and memory requirements"
+	help
+	  This is a meta-configuration option to significantly reduce the flash
+	  and memory requirements of the shell.  Enabling it will choose Kconfig
+	  defaults which favor reduced flash or memory requirements over extra
+	  features.
+
 config SHELL_STACK_SIZE
 	int "Shell thread stack size"
 	default 2520 if OPENTHREAD_SHELL
@@ -35,6 +43,7 @@ config SHELL_BACKSPACE_MODE_DELETE
 
 config SHELL_CMD_BUFF_SIZE
 	int "Shell command buffer size"
+	default 128 if SHELL_MINIMAL
 	default 256
 	help
 	  Maximum command size in bytes. One byte is reserved for the string
@@ -67,7 +76,7 @@ config SHELL_ARGC_MAX
 
 config SHELL_TAB
 	bool "Enable the Tab button support in shell"
-	default y
+	default y if !SHELL_MINIMAL
 	help
 	  Enable using the Tab button in the shell. The button
 	  can be used for prompting commands, or for autocompletion.
@@ -76,7 +85,7 @@ config SHELL_TAB
 config SHELL_TAB_AUTOCOMPLETION
 	bool "Enable commands autocompletion with the Tab button"
 	depends on SHELL_TAB
-	default y
+	default y if !SHELL_MINIMAL
 	help
 	  Enable commands and subcommands autocompletion with the Tab
 	  key. This function can be deactivated to save some flash.
@@ -84,7 +93,7 @@ config SHELL_TAB_AUTOCOMPLETION
 config SHELL_WILDCARD
 	bool "Enable wildcard support in shell"
 	select FNMATCH
-	default y
+	default y if !SHELL_MINIMAL
 	help
 	  Enables using wildcards: * and ? in the shell.
 
@@ -96,13 +105,13 @@ config SHELL_ECHO_STATUS
 
 config SHELL_VT100_COLORS
 	bool "Enable colors in shell"
-	default y
+	default y if !SHELL_MINIMAL
 	help
 	  If enabled VT100 colors are used in shell (e.g. print errors in red).
 
 config SHELL_METAKEYS
 	bool "Enable metakeys"
-	default y
+	default y if !SHELL_MINIMAL
 	help
 	  Enables shell meta keys: Ctrl+a, Ctrl+b, Ctrl+c, Ctrl+d, Ctrl+e,
 	  Ctrl+f, Ctrl+k, Ctrl+l, Ctrl+u, Ctrl+w, Alt+b, Alt+f
@@ -110,18 +119,18 @@ config SHELL_METAKEYS
 
 config SHELL_HELP
 	bool "Enable help message"
-	default y
+	default y if !SHELL_MINIMAL
 	help
 	  Enables formatting help message when requested with '-h' or '--help'.
 
 config SHELL_HELP_ON_WRONG_ARGUMENT_COUNT
 	bool "Enable printing help on wrong argument count"
 	depends on SHELL_HELP
-	default y
+	default y if !SHELL_MINIMAL
 
 config SHELL_HISTORY
 	bool "Enable history in shell"
-	default y
+	default y if !SHELL_MINIMAL
 	select RING_BUFFER
 	help
 	  Enable commands history. History can be accessed using up and down
@@ -129,6 +138,7 @@ config SHELL_HISTORY
 
 config SHELL_HISTORY_BUFFER
 	int "History buffer in bytes"
+	default 128 if SHELL_MINIMAL
 	default 512
 	depends on SHELL_HISTORY
 	help
@@ -136,18 +146,18 @@ config SHELL_HISTORY_BUFFER
 
 config SHELL_STATS
 	bool "Enable shell statistics"
-	default y
+	default y if !SHELL_MINIMAL
 
 config SHELL_CMDS
 	bool "Enable built-in commands"
-	default y
+	default y if !SHELL_MINIMAL
 	help
 	  Enable built-in commands like 'clear', 'history', etc.
 
 config SHELL_CMDS_RESIZE
 	bool "Enable resize command"
 	depends on SHELL_CMDS
-	default y
+	default y if !SHELL_MINIMAL
 	help
 	  By default shell assumes width of a terminal screen set to 80
 	  characters. Each time terminal screen width is changed resize command