Implement additions to initramfs-tools to support loop mounted root

Two kernel command line parameters are parsed and used:
looproot= specifies the block device that contains a loop mountable
drive, UUID is supported for this option
loopsrc= specifies the file on the looproot that will be used for loop
setup; this should be a *full* disk image including MBR

looproot/loopsrc will be setup as /dev/loop0 and its partitions will be
available through /dev/mapper/loop0pX, so to use the first partition as
the real root, one should specify root=/dev/mapper/loop0p1

ntfs-3g is hard coded for the looproot, and the appropriate
sendsigs.omit files are recorded for clean reboots, as these were not
being set by the provided local-bottom scripts.

@Motiejus, please review. I have tested the procedure in general, but don't know if simply including these files under includes.chroot will work. We might have to add a hook to regenerate initramfs.