wic/direct: reserve 2 sector for extended partition

extended partition should reserve at least 2 unallocated sectors
between the start of the extended partition and the start of a
logical partition.

[YOCTO #13658]

Signed-off-by: Chee Yang Lee <chee.yang.lee@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>