partitionedfs: add partial GPT support

partitionedfs: add partial GPT support

This commit partially adds GPT partitions support. Further patches will add
full support. I split them to make the patches more reviewable.

This patch basically implements most of the things except of actually asking
parted to use GPT partitioning.

This patch introduces another 'layout_partitions()' parameter which defines the
partition table format. It now may be 'msdos' or 'gpt'.

Change-Id: I210bacc629b938d307a1e4d209c90d4400a42daa
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>