Add extract_dtb

extract_dtb is a small utility to extract DTB files from Image.gz-dtb.
It searches the FDT header magic numbers in reversed order,
and output all found DTBs.

Test: extract_dtb Image.gz-dtb dtb Image.gz
Test: extract_dtb Image.gz-dtb dtb
Change-Id: I4e03291fd8ab5cf003f4d4cb2d153fbee56bfe7d