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: I8167cabff034e38e888c0957accf165bdd31673a