Add a script to unpack boot image

The script prints out the boot image header
contents and extracts the kernel, ramdisk,
second bootloader and recovery DTBO images.

Bug: 74763691
Test: unpack_bootimg --boot_img $OUT/recovery.img

Change-Id: Iadbca81c157d9e4607f808a14468ab5542347507
Merged-In: Iadbca81c157d9e4607f808a14468ab5542347507
(cherry picked from commit 02c08cc3471f11db10853c4f668d3348ce6f403f)