Fix "mach dtb" return code to avoid bogus boot

Fix "mach dtb" return code to avoid bogus boot

Bootloader command functions must return zero in case of failure,
returning 1 tells the bootloader to boot the currently set kernel iamge.

"machine dtb" is is the wrong way around so using it triggers a boot.

Fix this and print a brief usage (like other commands such as "hexdump" do)
while here.

Feedback OK patrick