init: enhance shared boot/recovery partition support via kernel cmdline

New BoardConfig options:
 * TARGET_NO_SEPARATE_RECOVERY - set to true when using a shared
   boot/recovery ramdisk.
 * BOARD_CHARGING_CMDLINE_VALUE_RECOVERY - value exported by kernel
   cmdline to signify that we are in recovery mode.

Change-Id: I475f0ff087493d9e7a69917296d2850049c0efd5