This makes a modified perst build.  Modified as in blkmmc0p14 is system and blkmmc0p27 is data.  This gives the presto 12g shared data/sdcard partition.
This also uses mtp instead of mass storage.

This will of course erase the internal sdcard.

When built, go into bootloader and fastboot flash recovery recovery.img first.
then boot into recovery. in recovery go to mounts and storage, and format /data and /data/media (sdcard) when done mount /data then from a computer go into adb shell

mkdir /data/media
mkdir /data/media/0
chown -R media_rw media_rw /data/media
chown -R media_rw media_rw /data/media/0

exit

now flash the build and gapps, and reboot into system

Note , it will take a few minutes for mtp to initialize and internal and sdcard to show up.