part: efi: Disable overlap check

The current code checks that no partitions overlap with the GPT partition
table using the offset of the first LBA usable for that partition.

This works fine, unless you have a partition entry that is further away
than it usually is and you want to create partitions in the gap between the
GPT header and the GPT partition entries, for example to reflash a
bootloader that needs to be set there.

Rework the test to something a bit smarter that checks whether a partition
would overlap with either the GPT header or the partition entries, no
matter where it is on the disk.

Partitions that do not have a start LBA specified will still start at the
first LBA usable set in the GPT header, to avoid weird behaviours.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>