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