init: add charge mode handling

Introduces a 'charger' section that is processed when androidboot.mode
supplied on the kernel commandline is "charger".

In this mode, sections such as fs, post-fs, etc are skipped. Only the
'early-init' and 'init' sections of the init rc files are processed before
processing the 'charger' section.

Change-Id: If9eb6334de18f04cbcf2aab784578e2993615242
Signed-off-by: Dima Zavin <dima@android.com>