shellcheck: enable testing

openbmc-build-scripts change Ie25c27cffc96b7fa66291abef1b0f90a0f1862ed
added support for running `shellcheck` as a lint tool for shell scripts.
Enable it here.

Signed-off-by: Patrick Williams <[email protected]>
Change-Id: I74ef500e5353913c79c90c7e03b1a75f4399ccef

Author: williamspatrick