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

Conflicts:
	init/Android.mk
init: Set the "ro.boot.emmc" property

 * QC switched to this from the old "ro.emmc" property.
 * Set both values since many things depend on this.

Change-Id: I958dbb13b08bd576ad16318683a0eac45dfc6fee
init: Add an emmc-specific configuration file

 * Load init.emmc.rc if emmc_boot is set
 * This makes startup much cleaner for devices with a mess of vendor RC
   configurations.

Change-Id: Ifa6d0ca104c11e037740690da0ff6284c2f4a3ca
fix up adb to prefer /sbin/sh if it exists

Change-Id: Ifa68efec387e6c12dc4bb2d0e764b8ecf96c0ad5