Support alternative .android_secure location

Some devices (jewel, m8) have .android_secure at a non-standard
location. Allow a different partition to be specified as the home of
.android_secure using the fstab flag 'andsec'.

Using the /data/media variant of jewel as an example, recovery.fstab or
twrp.fstab could be edited like follows:

/external_sd  vfat  /dev/block/mmcblk1p1  flags=display="MicroSD";andsec

Note, this is NOT a method to move .android_secure in your ROM. Your ROM
must already write to this alternative location.

Change-Id: I3a6e4e63aaddb35870b79e80938b0f9c2c902443