init: Support for emmc boot and init.target.rc

 * Via CodeAurora

Squashed an additional commit in as it's closely related:
Deprecate distinct boot paths for androidboot.emmc

Qualcomm used to require a "on emmc-fs" to distinguish multi-boot
devices, in replacement of the usual "on fs". This distinction has
been deprecated in jb, and it's breaking mako

Change-Id: Iaa9ed356574f364d8061e8941cffc3d458efc603