mkefidisk: Unmount after partitioning

mkefidisk: Unmount after partitioning

Some automounters are rather overzealous and like to mount things
immediately after partitioning. This can happen if the disk is being
reused and the partitions align exactly with the existing partitions
which have already been formatted. Move the unmount code into a function
and call it before and after partitioning.

(From OE-Core rev: f1854e458e5e77806b1fc837033500fa91272261)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>