build-sgiboot: reorganize the disk preparation code

The disk preparation code is reorganized to make it more readable.
In addition to this, the functions are now parameterized and the
ext3 partition gets a UUID assigned to it for use by grub.

Change-Id: Icc8d380a7410de293c44aedcd705ed6a123da42d
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>