boot: Move SYS_BOOTM_LEN to be by LEGACY_IMAGE_FORMAT

This particular option is required for booting all image types,
regardless of if we are starting an OS via command line or something
else.  Move the question for SYS_BOOTM_LEN to be by the question for
LEGACY_IMAGE_FORMAT, as that's where our generic OS questions start.

Signed-off-by: Tom Rini <trini@konsulko.com>