board/common: mnt: Correctly reset /var as part of a factory-reset

Before this change, the following would happen in /lib/infix/mnt if a
factory-reset was performed:

1. Copy the contents of /var from Squash into /mnt/var and bind mount
   /mnt/var over /var
2. Check for factory-reset
3. Clear /mnt/cfg/* and /mnt/var/*
4. Mount overlays

This ordering leaves the system with a completely empty /var on the
boot when a factory-reset is executed. Finit will fixup some of this
via its tmpfiles scripts, but we want the ability to ship files under
/var as part of the Squash and have these be available after the bind
mount, just like what happens at every other boot, when we don't
perform a factory-reset.

We solve this by delaying the bind mount (1) until after the reset has
been performed, together with all the overlays (4). While we're here,
make the fallback case, where no persistent /var is available, use a
bind mount as well. This should allow containers to be tested on such
setups, and it's one less flavor to test.

New order:

1. Check for factory-reset
2. Clear /mnt/cfg/* and /mnt/var/*
3. Mount overlays
4. Copy the contents of /var from Squash into /mnt/var and bind mount
   /mnt/var over /var

Now the only difference between a regular boot and a
"factory-reset-boot" is whether (2) executed or not.

board/common/rootfs/lib/infix/mnt

@@ -109,12 +109,8 @@ varsrc=/mnt/var
 if ! mount_rw var >/dev/null 2>&1; then
     logger $opt -p user.warn -t "$nm" \
 	   "No persistent storage found for /var, only /var/lib is persisted."
-    varsrc=/mnt/tmp
+    varsrc=/mnt/tmp/infix/var
     vlibsrc=/mnt/cfg/vlib
-else
-    mount_bind "$varsrc" /var
-    varsrc=
-    vlibsrc=
 fi
 
 cfgsrc=/mnt/cfg
@@ -145,9 +141,9 @@ mount_overlay cfg  $cfgsrc /cfg
 mount_overlay etc  $etcsrc /etc
 mount_overlay home $cfgsrc /home
 mount_overlay root $cfgsrc /root
+mount_bind         "$varsrc" /var
 
-[ "$varsrc" ]  && mount_overlay var "$varsrc"  /var
-[ "$vlibsrc" ] && mount_bind        "$vlibsrc" /var/lib
+[ "$vlibsrc" ] && mount_bind "$vlibsrc" /var/lib
 
 # Keep transient configs in a ramdisk and symlink out to /cfg for
 # startup.