mkbootimg: support boot image version 4

Support packaging multiple ramdisk fragments in the vendor boot image.

Bug: 162864255
Test: manually create a v4 boot image and verify with the output \
  of unpack_bootimg
Test: mkbootimg --vendor_boot <> --header_version 4 --dtb <> \
  --ramdisk_type platform --board_id0 0x0100 --vendor_ramdisk_fragment <> \
  --ramdisk_type recovery --board_id0 0x0100 --vendor_ramdisk_fragment <> \
  --board_id0 0x0200 --board_id15 0x0215 --vendor_ramdisk_fragment <> \
Test: mkbootimg -h
Change-Id: Ida6c7c6d9d9722dce59b96ffb33c56882ca1e25b