Update the docs with details on how extracting the ISO contents works.

This also documents the new .extract make targets and moves the direct
loopback mount option entirely into the documentation.  It has been
moved because I want to discourage people from using it - I consider
it to be a higher risk with no good return for this risk.

Makefile

@@ -275,15 +275,6 @@ $(DEPSDIR)/slice.insert.deps: Makefile
 %.dosflash.exe.orig: %.iso.orig
 	mcopy -m -i $^@@$(FAT_OFFSET) ::FLASH/DOSFLASH.EXE $@
 
-MOUNT_POINT ?= /mnt
-MOUNT_OPTIONS ?= loop,ro
-
-# Mount the "embedded" fat file system from a given iso.
-%.iso.mount: %.iso
-	sudo mount -o $(MOUNT_OPTIONS),offset=$(FAT_OFFSET) $< $(MOUNT_POINT)
-%.iso.orig.mount: %.iso.orig
-	sudo mount -o $(MOUNT_OPTIONS),offset=$(FAT_OFFSET) $< $(MOUNT_POINT)
-
 # Extract the "embedded" fat file system from a given iso.
 %.iso.extract: %.iso
 	mcopy -s -i $^@@$(FAT_OFFSET) :: $@

docs/HACKING.txt

@@ -21,8 +21,8 @@ Then start radare with one of these projects:
 
     radare -p x230.G2HT35WW
 
-Makefile targets:
------------------
+Makefile targets for patching:
+------------------------------
 
 If you expect to do development work, the following make targets are
 probably useful:
@@ -65,6 +65,42 @@ probably useful:
 
         make patched.x230.img.test
 
+Extracting the ISO images:
+--------------------------
+
+While examining a new ISO image, it is often useful to extract its
+entire contents.  Due to the way these images are constructed, it is
+not straight forward to get the data out.
+
+The Lenovo ISO image is a iso9660 filesystem image with /no/ files
+in it!  There is a El-Torrito standard boot descriptor in this image,
+and it defines a hard drive image for booting.  This hard drive image
+is a DOS MBR partitioned disk, with one partition.  The partition is a
+FAT16 filesystem, containing the actual files used in the BIOS update.
+
+To access the FAT16 filesystem, the byte offset of the filesystem
+is calculated (via a process of unwrapping and decoding each layers
+described above).  So far, every ISO image file from Lenovo that has been
+checked has used the same offset, so this has been hardcoded in the
+Makefile as the "FAT_OFFSET" variable.
+
+To extract the files for examination, a make target is available which
+can be used on any *.iso.orig or *.iso file.  e.g:
+
+    make g2uj25us.iso.orig.extract
+    make patched.x230.iso.extract
+
+In the case where a file needs to be modified inside the image, it is
+highly recommended to use the mtools "mcopy" utility to copy files back
+into the image - this is the safest method and does not require any
+elevated priviledges or run the risks of mounting foreign filesystems.
+
+If you find you do need to directly mount the filesystem, the following
+command can be used as a starting point:
+
+    sudo mount -o loop,offset=$FAT_OFFSET $ISOFILE $MNTPNT
+
+
 References:
 -----------
     http://forum.thinkpads.com/viewtopic.php?f=69&t=120776