Fix replicant dismounting of /usr/src
Fix replicant dismounting of /usr/src

There was a logic flaw in the dismount of replicant's /usr/src.  It
checked if /usr/src existed, not if $SYSTEM_ROOT/usr/src existed.  This
caused an error in the 05_* logic but prehaps there was no other issue
with that.

jrmarino/synth,jrmarino/synth,jrmarino/synth,jrmarino/synth