m7-common: use by-name paths for partitions in fstab

Change-Id: I0575efd29d31359ca9243e8967e919b509776575
m7-common: disable secure discard for ext4 wipes

Users are reporting really long wipe times in recovery on ext4
partitions.

Change-Id: I92dc84b2912684745521176bdb3d61d83e3d4130