package/mender: fix missing /var/lib

Mender needs /var/lib directory to be available: on some configurations
/var/lib is not available and thus the mender package installation fails.
This patch does a mkdir to ensure the /var/lib directory is always
available.

Fixes:

  http://autobuild.buildroot.net/results/d2237083a13ab7688dd2b6dc8dbcd4226ed5651a/



Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>