kernel: Add support for vendor_boot image header version 4

vendor_boot v4 supports packaging multiple "fragmented" vendor ramdisks.
A secondary can be used to store the kernel modules instead of storing
them in the "normal" vendor_ramdisk.

Test: Create a vendor_boot with source built kernel/modules,
      gs101 still boots.
Change-Id: Ic295ccf201c2593e08b9559ae28f80c70a7eb341