Adding vendor_boot-debug.img

Commit I30137c3caef91805d9143d404e5e4d06c0fccc30 adds boot-debug.img
to allow adb root with an user build GSI image.

  https://source.android.com/compatibility/vts/vts-on-gsi

Another commit I5b005097b73f59857c3a2f92d693b3e67ee8424e adds
vendor_boot.img to pair with a generic kernel image, the GKI boot.img.

To allow adb root for devices using a GKI, vendor_boot-debug.img is
introduced. The image combination used in VTS will be:

    Old devices without GKI:
      GSI system.img + boot-debug.img + vendor.img, etc.

    New devices with GKI:
      GSI system.img + GKI boot.img + vendor_boot-debug.img + vendor.img, etc.

Note that boot-debug.img still can be used on new devices for
non-compliance scenario.

Bug: 147849477
Test: lunch aosp_cf_x86_64_phone-userdebug; make vendorbootimage_debug
Test: `make dist`, checks that both vendor_boot-debug.img and
      vendor-ramdisk-debug.cpio.gz are in $OUT/ and out/dist.
Test: `make dist`, checks that installed-files-vendor-ramdisk-debug.{json,txt} are
       in $OUT/ and out/dist.
Change-Id: I66b662d8b1e5c619ed7bb81e40233fe9df363b27