hammerhead: add charging led support

Also introduces led priority handling for
battery vs notification vs attention
(copied from jf-common).

Requires the hammerhead kernel led
always on support patch.

Change-Id: I1a2f3aae457c6d7e4e3c3f9ca39cbe42d39c3bc2 (reverted from commit 02fb3ea5a89fcb0b2f597f31b5e5266d2025b50b)