km: update the boot script to check for a DTB

km: update the boot script to check for a DTB

If a DTB is found with cramfsls, the bootscript continues as expected.
If none is found, the cramfsloadfdt and boot subbootcmds are updated to
not load the DTB from cramfs and not pass it to the kernel. The kernel
thus must have an appended DTB otherwise the boot will fail.

This is required for the km_kirkwood boards that must support .esw where
the DTB sometimes is appended (for backwards compatibility) and sometimes
is passed correctly (as we do now for all newer boards).

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Reviewed-by: Heiko Schocher <hs@denx.de>