obmc-flash-bmc: fix shellcheck warnings

Tested: Built the ubi.mtd.tar file and loaded it onto a witherspoon
machine, verified ubi volumes were updated and mounted after reboot as
expected. Performed a code update on mmc system, verified it booted as
expected.

Signed-off-by: Isaac Kurth <blisaac91@gmail.com>
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Change-Id: I69cde49626c0a7f8cf8d0028b2c24d2c3ca4458e
1 file changed