tizen: sd_fusing.py: add support for partition start & size specified in sectors

tizen: sd_fusing.py: add support for partition start & size specified in sectors

Add support for specifying partition's start & size in sectors and adjust
lba-start parameter (first allowed sector of the first partition) to 34,
what is the minimal possible value for the GPT layout. This doesn't affect
any start or size of the partitions specified in MBytes, because according
to sfdisk manual, they are always aligned to 1MB.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I8fdccb896072a9cddfe7e185f6650eb0471caae2