Move bootimage props to vendor/build.prop

Prior to this change the properties were in system/etc/default.prop.
These properties are device-specific and don't really belong on the
/system partition.

I anticipate further change to these properties in the future:
- pruning down the set of properties, as the .product. props
  don't make much sense for the boot image
- moving them to the ramdisk instead

Bug: 130025216
Test: boot into recovery, observe title (shows bootimage fingerprint)
Change-Id: I9e92c1ec7068ae18fa0d709c77eac22a6b88c3d8