Support building recovery as standalone ramdisk in vendor_boot v4

If BOARD_BOOT_HEADER_VERSION >= 4,
BOARD_MOVE_RECOVERY_RESOURCES_TO_VENDOR_BOOT is true and
BOARD_INCLUDE_RECOVERY_RAMDISK_IN_VENDOR_BOOT is true, then build
recovery as a standalone ramdisk fragment in vendor_boot image.

The recovery ramdisk would be a vendor ramdisk fragment packaged in
vendor_boot, whose ramdisk_name is "recovery" and ramdisk_type is
"VENDOR_RAMDISK_TYPE_RECOVERY".

Bootloader can omit loading the recovery ramdisk during normal boot to
optimize the size of the initramfs.

Bug: 183395459
Test: Presubmit
Test: Modify BoardConfig of CF and m dist. Verify the vendor_boot.img
  with unpack_bootimg.
Test: Strip the vendor_boot of the recovery ramdisk, and verify that CF
  can boot to normal boot without the recovery ramdisk.
Change-Id: I6e9a2781ec87aece10d4844fa18bbe9a7b4674e6