Allow specifying kernel toolchain and prefix

Change-Id: Ib75cfe772e073f2196455f3e3dfba247f6b3feff
build: recovery: Support adding device-specific items

(cdesai): Also add the relevant parts from koush's
commit 7fd5a89df5ec3961c9aedfd2afad50d6d2561056
"Add initial support for charge mode. also support recovery/root in the device tree to easily copy props into recovery images."

Change-Id: I25205e68282680932917016646dabd1abadbfce6
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