Specify --ramdisk_name for vendor ramdisks for vendor_boot v4

When building vendor_boot v4 with multiple vendor ramdisks, pass the
name of each ramdisk to mkbootimg.

Bug: 177882072
Test: Modify a device BoardConfig and verify the built vendor_boot.img
  with unpack_bootimg
Change-Id: I679d277aa88f1e10678d2f74b579e4140536e267