Add support for building "foreign" images, ie non host arch images. This make it...

Add support for building "foreign" images, ie non host arch images. This make it possible to use vmdebootstrap to create Raspberry Pi images. It add support for these new options:

  --bootsize size   when not the defautl '0%', create /boot/ partition
    	     	    of the given size

  --boottype fstype file system type of /boot/ (default ext2)

  --foreign /path/to/binfmt_handler
                    When set, use foreign support in debootstrap.

  --variant variant ask debootstrap to build variant, not the
  	    	    default base system.

  --no-extlinux     skip installation of extlinux, depend on customize
                    script to make image bootable.

With this patch in place, I got a working Raspberry PI image by adding
a small customize script to install the binary blob needed to boot the
Raspberry Pi.