Add support for packing multiple vendor ramdisks in vendor_boot

Add support for partitioning the vendor_boot kernel modules into
multiple vendor ramdisk fragments. The partition granularity is kernel
module directory. This mechanism builds upon the existing
BOARD_KERNEL_MODULE_DIRS mechanism. For example, say we have three
kernel module directories:

BOARD_KERNEL_MODULE_DIRS := foo bar baz

We can then define a vendor ramdisk fragment:

BOARD_MKBOOTIMG_ARGS += --header_version 4
BOARD_VENDOR_RAMDISK_FRAGMENTS := dlkm_foobar

And let said ramdisk to contain the DLKM directories "foo" and "bar":

BOARD_VENDOR_RAMDISK_FRAGMENT.dlkm_foobar.KERNEL_MODULE_DIRS := foo bar
BOARD_VENDOR_RAMDISK_FRAGMENT.dlkm_foobar.MKBOOTIMG_ARGS := <mkbootimg args>

The built vendor_boot image would contain two ramdisks.
The first one being the "default" ramdisk, which contains DLKM directory
"baz" and the rest of the files that get's installed to
$(TARGET_VENDOR_RAMDISK_OUT).
The second one is the "dlkm_foobar" ramdisk, which contains the two DLKM
directories.

Design doc: go/vendor-boot-v4

Bug: 162864255
Test: Modify BoardConfig.mk to have a product build v4 vendor_boot
Test: Use unpack_bootimg to verify the vendor_boot image
Test: Teach a bootloader how to handle v4 boot image, flash boot &
  vendor_boot and boot device
Change-Id: Ibb1bbd7ebe36430c55ec6c4818c1d3888a319089