recovery: align /sdcard and /emmc with running system

By default, the recovery will create a symlink for /sdcard pointing to
/storage/sdcard0 and leave /emmc as an empty directory unconnected to
the vold-managed volumes.  For historical reasons, we use
/storage/sdcard1 as our /sdcard storage; set up the /sdcard and /emmc
links accordingly in postrecoveryboot.sh, which allows the CM updater to
work correctly.

Change-Id: Iea00ddc056c7caee591127be53fc20d2a6b12e7a