add external_sd directory to ramdisk for mounting external sdcard in cwmr
This addition allows external sdcard mounting in recovery by providing the necessary mountpoint.

Change-Id: Iad32e531d9d35b370645dcf729bc5a70cbf21167
Merge branch 'cyanogenmod' into cm-10.1

Signed-off-by: Humberto Borba <humberos@gmail.com>