s4-common: Create firmware symlinks at build time

Change-Id: I25af751f22ded9d438b304cbe079acef9f0ed930
s4-common: disable secure discard for ext4 wipes

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

Change-Id: I2f326622917266e9f43ea12da85889ca843008ca