liveusb: enable running OpenELEC from only one USB stick

possible syslinux menu with option for running installer or live system
new mount type FILE with optional size in MB for loopback file
  disk=FILE=storage.loop
  disk=FILE=storage.loop,100

dd and mkfs.ext4 are used from system (no need to increase the size of initramfs)

packages/initramfs/sysutils/busybox-initramfs/scripts/init

@@ -36,6 +36,7 @@
   IMAGE_KERNEL="KERNEL"
   IMAGE_SYSTEM="SYSTEM"
 
+  BOOT_STEP="start"
   REBOOT="0"
   MD5_FAILED="0"
   MD5_NOCHECK="0"
@@ -43,6 +44,7 @@
 
   NBD_DEVS="0"
   FLASH_FREE_MIN="5"
+  LIVE_FLASH_FREE_MIN="50"
 
   INSTALLED_MEMORY=`cat /proc/meminfo | grep 'MemTotal:' | awk '{print $2}'`
   SYSTEM_TORAM_LIMIT=1024000
@@ -292,6 +294,9 @@
       NFS=*)
         MOUNT_CMD="mount_nfs"
         ;;
+      FILE=*)
+        MOUNT_CMD="mount_common"
+        ;;
       *)
         error "mount_part" "Unknown filesystem $1"
         ;;
@@ -304,7 +309,10 @@
       echo "updating $1..."
         $IONICE /bin/busybox mount -o remount,rw /flash
         $IONICE /bin/busybox mv $UPDATE_DIR/$2 $3
+        # loopback file needs writable /flash all the time
+        if [ "${disk%%=*}" != "FILE" ]; then
           $IONICE /bin/busybox mount -o remount,ro /flash
+        fi
         $IONICE /bin/busybox sync
     fi
   }
@@ -327,7 +335,6 @@
     fi
   }
 
-
   hfsdiskprep() {
     for DEVICE in /dev/sd*; do
       for device in $(/bin/busybox blkid $DEVICE); do
@@ -398,6 +405,65 @@
     progress "Mounting storage"
 
     if [ -n "$disk" ]; then
+      if [ "${disk%%=*}" = "FILE" ]; then
+        target="${disk%%,*}"
+        storage_loop_file="/flash/${target#*=}"
+        $IONICE /bin/busybox mount -o remount,rw /flash
+
+        if [ ! -f "$storage_loop_file" ]; then
+          echo "Creating storage loopback file: $storage_loop_file..."
+          STORAGE_USE=$(/bin/busybox df /flash/ | awk '/[0-9]%/{print $4}')
+          STORAGE_USE=$(($STORAGE_USE / 1024 - $LIVE_FLASH_FREE_MIN))
+
+          options="${disk#*,}"
+          if [ "$options" = "$disk" ]; then
+            echo "No size specified, using all available space..."
+            if [ $STORAGE_USE -gt 0 ]; then
+              echo "Size obtain: $STORAGE_USE MB..."
+            else
+              error "size check" "Not enough free space (at least $LIVE_FLASH_FREE_MIN MB)..."
+            fi
+          else
+            if [ $options -le $STORAGE_USE ]; then
+              STORAGE_USE=$options
+              echo "Size specified: $STORAGE_USE MB..."
+            else
+              echo "Incorrect specified size: $options MB > $STORAGE_USE MB..."
+              if [ $STORAGE_USE -gt 0 ]; then
+                echo "Size trimmed: $STORAGE_USE MB..."
+              else
+                error "size check" "Not enough free space (at least $LIVE_FLASH_FREE_MIN MB)..."
+              fi
+            fi
+          fi
+
+          if [ -f "/flash/$IMAGE_SYSTEM" ]; then
+            # /flash is filesystem with system image file
+            # use dd and mkfs.ext4 from system
+            mount_part "/flash/$IMAGE_SYSTEM" "/sysroot" "ro,loop"
+            echo "Creating empty file..."
+            /sysroot/bin/busybox dd if=/dev/zero of="$storage_loop_file" bs=1M count=$STORAGE_USE &>/dev/null
+            echo "Formating to EXT4 filesystem..."
+            LD_LIBRARY_PATH=/sysroot/usr/lib /sysroot/usr/bin/mkfs.ext4 -F -L StorageLive "$storage_loop_file" &>/dev/null
+            /bin/busybox sync
+            umount /sysroot
+          else
+            # /flash is actual root filesystem
+            echo "Creating empty file..."
+            /flash/bin/busybox dd if=/dev/zero of="$storage_loop_file" bs=1M count=$STORAGE_USE &>/dev/null
+            echo "Formating to EXT4 filesystem..."
+            LD_LIBRARY_PATH=/flash/usr/lib /flash/usr/bin/mkfs.ext4 -F -L StorageLive "$storage_loop_file" &>/dev/null
+            /bin/busybox sync
+          fi
+
+          echo "Done..."
+          /bin/busybox usleep 2000000
+        fi
+
+        mount_part "FILE=$storage_loop_file" "/storage" "loop,rw,noatime"
+        return
+      fi
+
       if [ -n "$OVERLAY" ]; then
         OVERLAY_DIR=`cat /sys/class/net/eth0/address | /bin/busybox tr -d :`
 
@@ -496,7 +562,8 @@
     fi
 
     if test "$REBOOT" -eq "1"; then
-      echo "System reboots now..." && \
+      echo "System reboots now..."
+      /bin/busybox usleep 2000000
       /bin/busybox reboot
     fi
   }
@@ -526,6 +593,10 @@
     [ -f "/sysroot/sbin/init" ] || error "final_check" "Could not find system."
   }
 
+  if [ "${boot%%=*}" = "FILE" ]; then
+    error "check arguments" "boot argument can't be FILE type..."
+  fi
+
 # main boot sequence
   for BOOT_STEP in \
       load_modules \