mount-usb: replace fixed timeout with drive detection

Rather than wait a fixed 5s for the usb storage kernel modules
to load, and the user to insert a drive, check for new USB drives
inserted every 1s with a 5s timeout.

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>

initrd/bin/mount-usb

@@ -5,9 +5,16 @@
 enable_usb
 
 if ! lsmod | grep -q usb_storage; then
-  insmod /lib/modules/usb-storage.ko \
+  count=$(ls /dev/sd* 2>/dev/null | wc -l)
+  timeout=0
+  echo "Scanning for USB storage devices..."
+  insmod /lib/modules/usb-storage.ko >/dev/null 2>&1 \
   || die "usb_storage: module load failed"
-  sleep 5
+  while [[ $count == $(ls /dev/sd* 2>/dev/null | wc -l) ]]; do 
+    [[ $timeout -ge 4 ]] && break
+    sleep 1
+    timeout=$(($timeout+1))
+  done
 fi
 
 if [ ! -d /media ]; then