Allow devices to specify certain ro. props via TARGET_UNIFIED_DEVICE

Thanks dhacker for pointing this out to me.....

This modifies buildinfo.sh to not set model, name, description, and
fingerprint in build.prop. These can then be set via custom init
lib.
This also fixes ota_from_target_files to function without these
props being preset.

Change-Id: I945b2c38e60b207d2d0f82b34f4f230ac21b0657

Conflicts:
	core/Makefile
	tools/buildinfo.sh
	tools/releasetools/ota_from_target_files