unpack_bootimg: Refactor unpack_vendor_boot_image()

Straighten out the code of unpack_vendor_boot_image(), factor out common
code to a helper class. Goal is to make the code easier to follow and to
extend.

Bug: 180712476
Test: atest --host mkbootimg_test
Change-Id: Iae3b84050f44b16b179972fa7395bb14689c98f9
1 file changed