init: Bring back support for arbitrary chargermode cmdlines

This was accidentally removed while debugging the init breakage
fixed in change I8ef5c932efcd5e7f8d6f6fce0915683d84c3ee11. Put
it back

Change-Id: If35b9d5db281f8aab6de26f2bb63717d28b2db30
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
Merge remote-tracking branch 'cyanogen/cm-12.0' into cm-12.0