This will make a separated section for dtb appear in ELF, and we can
then use objcopy to patch a dtb into vmlinuz when RAW_APPENDED_DTB
is set in kernel config.

command to patch a dtb:
objcopy --set-section-flags=.appended_dtb=alloc,contents \
        --update-section=.appended_dtb=<target>.dtb vmlinuz
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>