mkefidisk.sh: Add die() and cleanup() routines

mkefidisk.sh: Add die() and cleanup() routines

Currently the script will attempt to continue even after a fatal error.
Add a die() routine which will abort in the case of a fatal error and
call a cleanup() routine to unmount any images or devices and remove the
TMPDIR.

Move the variable assignment and directory creation earlier in the
script, making it more obvious what we need to clean up.

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