board/common: factory reset using shred and LED indication

This patch greatly improves the security of the Infix factory reset by
replacing 'rm' with 'shred'.  The shred tool overwrites the contents of
files three times.  With the -z and -u options the last pass writes
zeroes and then uninks the files.

On the NanoPi R2S the factory-reset now takes 24 seconds:

   Aug 11 16:00:34 infix mnt[121]: Resetting to factory defaults.
   Aug 11 16:00:58 infix mnt[173]: Factory reset complete.

A visual aid is also added, setting *all* LEDs to on, that can be found
mounted on the device, before starting the file shredders.  The LEDs are
reset back to off and SYS red blinking as soon as the wipe has completed
and iitod has been started.

Fixes #158

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

board/common/rootfs/usr/libexec/infix/mnt

@@ -42,9 +42,17 @@ check_factory()
 
 factory_reset()
 {
-    # XXX: flash LEDs to confirm factory-reset in progress
+    find /sys/class/leds/ -type l -exec sh -c 'echo 100 > $0/brightness' {} \;
     logger $opt -p user.crit -t "$nm" "Resetting to factory defaults."
+
+    # Shred all files to prevent restoring contents
+    find /mnt/cfg -type f -exec shred -zu {} \;
+    find /mnt/var -type f -exec shred -zu {} \;
+
+    # Remove any lingering directories and symlinks as well
     rm -rf /mnt/cfg/* /mnt/var/*
+
+    logger $opt -p user.crit -t "$nm" "Factory reset complete."
     sync
 }