Allow override of device asserts, including multi-device support.

Set in board file with TARGET_OTA_ASSERT_DEVICE.
(cherry-picked from commit 0f452f21fc9323b9d1fe746161761cf40aaa5030)

Change-Id: I3d06bdc0e3e26bde0c0e646accd050364f9713b9

Conflicts:
	core/Makefile
	tools/releasetools/ota_from_target_files
Allow devices to specify certain ro. props via TARGET_UNIFIED_DEVICE

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/releasetools/ota_from_target_files
Merge branch 'lollipop_dev' of github.com:AOSParadox/android_build into lollipop_dev

Conflicts:
	core/Makefile