kernel-resin.bbclass: Fix getVar() invocation
kernel-resin.bbclass: Fix getVar() invocation

Since poky c7610aa353e31977c099ce574acb188a3f4219e0 , default expand=False has been dropped.
Hence, we need to explictily set the expand parameter in order to fix the following error:

Exception: TypeError: getVar() takes at least 3 arguments (2 given)

Signed-off-by: Florin Sarbu <3a837a37a2ac9072a088641ec330e0e5d5b86f4e@resin.io>

resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin