x86: preserve partition table on sysupgrade

With this patch sysupgrade will write directly to the partitions
instead of to the main disk.  The UUID is copied from the image
to the MBR as well.  This prevents the mbr from being completely
overwritten and losing the partition table.  The -p option has
been added to maintain the original behavior and overwite the
entire disk with the new image.  Tests have been added to ensure
that the image partitions match up with the active partitions.

Signed-off-by: Rob Mosher <nyt-openwrt@countercultured.net>

SVN-Revision: 48682