boot: Create a new CONFIG_BOOT option

At present we assume that the cmdline is enabled, so CMD_BOOTM is used
to determine the presence of the boot_run() function.

As a first step towards enabling boot_run() without CONFIG_CMDLINE,
create a new BOOT option. Select it in BOOT_DEFAULTS and make CMD_BOOTM
depend on it. Make it default y to mirror the bootm command.

The intention with this option is that it covers the core code used by
all boot commands (e.g. zboot, bootm, booti). This includes
bootm_run_states() and functions called from there.

The name 'BOOT' is used instead of 'BOOTM', since bootm is actually a
command and it may be that we might wish, for example, to enable the
'zboot' part of the boot functionality, without enabling the 'bootm'
part of it.

Signed-off-by: Simon Glass <sjg@chromium.org>

boot/Kconfig

@@ -385,6 +385,14 @@ endif # VPL
 
 endif # FIT
 
+config BOOT
+	bool
+	default y
+	help
+	  Enables the main boot functionality, called 'bootm' in the code. This
+	  is used by commands like bootm, bootz, and booti to effect a boot.
+	  If CMDLINE is disabled, booting is still possible via standard boot.
+
 config PXE_UTILS
 	bool
 	select MENU
@@ -419,6 +427,7 @@ config BOOT_DEFAULTS
 	bool  # Common defaults for standard boot and distroboot
 	select BOOT_DEFAULTS_FEATURES
 	select BOOT_DEFAULTS_CMDS if CMDLINE
+	select BOOT
 	help
 	  These are not required but are commonly needed to support a good
 	  selection of booting methods. Enable this to improve the capability