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/cmd/Kconfig b/cmd/Kconfig
index 748b959..16822c5 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -219,6 +219,7 @@
 
 config CMD_BOOTM
 	bool "bootm"
+	depends on BOOTM
 	default y
 	help
 	  Boot an application image from the memory.