hammerhead: TWRP-ify

Change-Id: I8dd3a05b98c1e3c6db46a52f7e33c7a5aefd7d1b
Finalize the TWRP-ify

Copy the twrp.fstab to the recovery ramdisk

Change-Id: I8c39f1dde79a1005079f1dd2c41163efde5bf353