core-utils: Add generate_vendor_boot_image subcommand

- Provides subcommand 'generate_vendor_boot_image'
  to generate vendor_boot image.
- generate_vendor_bundle command now requires
  PRODUCT_OUT to be passed to it.

Change-Id: I91b59181f8372e7d7a6e63fe4a42dac428aca081
2 files changed