init: Add support for Samsung's bootchg command line

Samsung's pre-kitkat bootloader used androidboot.bootchg=true|false
to indicate offmode charging or not. With the kitkat bootloaders,
they have switched to the AOSP way of using androidboot.mode=charger
to indicate that offmode charging mode is active. This will then
trigger the charger class.

With this patch, we can get rid of lpm.rc and init.qcom.lpm_boot.sh,
and instead use the charger class to support offmode charging. This
way we can support offmode charging for both pre-kitkat and
kitkat bootloaders at the same time.

Change-Id: Ida8f4327e5a5fbf07e7ee44586461ddef33431be