init: add detection of charging mode

Based on Prashant Somashekar's patch which is in turn based on techomancer's
patch http://goo.gl/I19GG.

When BOARD_CHARGING_MODE_BOOTING_LPM is set, init will read that
value (usually from /sys) and enable charging mode when set to 1.

This differs from the original patch in that Samsung-specific lpm.rc
is not loaded, but instead, init.rc is loaded using the "charger" class,
which is how AOSP devices handle charging.

The change is because the either the SELinux or the init.rc changes is
causing lpm.rc to not work, and porting lpm.rc to the Android system
is easier than trying to fix it.

Change-Id: Ice7a074da7e982f9a64aaa97de28bfcbccdc8b91