Support vendor bootconfig creation

Gather all BOARD_BOOTCONFIG parameters.
Create vendor-bootconfig.img with parameters seperated by newlines. Pass
that file to mkbootimg as --vendor_bootconfig to add it to the
vendor_boot.img.

Test: Add BOARD_BOOTCONFIG parameters in cuttlefish .mk file
      Check vendor-bootconfig.img for expected output
      Verify expected vendor_boot.img format with:
      unpack_bootimg --boot_image vendor_boot.img
Test: Update Cuttlefish bootloader to handle the new vendor_boot.img and
      check /proc/bootconfig for the expexted parameters.
Bug: 173815685
Change-Id: Iaa9b71b4bc64375777a5353396e83bb2beb25c47