bootm: Add a Kconfig option for bootm functionality

Create a separate Kconfig option which enables the bootm logic,
separate from the 'bootm' command. This will eventually allow booting
without CMDLINE enabled.

Update boards which disable CMD_BOOTM to disable BOOTM instead, since
CMD_BOOTM now depends on BOOTM

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/colibri_vf_defconfig b/configs/colibri_vf_defconfig
index 8b2324f..48019f0 100644
--- a/configs/colibri_vf_defconfig
+++ b/configs/colibri_vf_defconfig
@@ -17,6 +17,7 @@
 CONFIG_ENV_VARS_UBOOT_CONFIG=y
 CONFIG_HAS_BOARD_SIZE_LIMIT=y
 CONFIG_BOARD_SIZE_LIMIT=520192
+# CONFIG_BOOTM is not set
 CONFIG_BOOTDELAY=1
 CONFIG_FDT_FIXUP_PARTITIONS=y
 CONFIG_USE_BOOTCOMMAND=y
@@ -33,7 +34,6 @@
 CONFIG_SYS_PROMPT="Colibri VFxx # "
 CONFIG_SYS_PBSIZE=1056
 # CONFIG_CMD_BOOTD is not set
-# CONFIG_CMD_BOOTM is not set
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_IMI is not set