Commit b36056af20e981433f143556d511dec5644930fc in oe-core master
(matching commit in poky master is a40f5d34065129ab19a3c981cbb38f854e9ae0ec)
moved some bits out of u-boot.inc (from oe-core) into u-boot-common_${PV}.inc. This
breaks many u-boot recipes in the wild, including u-boot_2016.09.01 from this layer
since the variable S itself results undefined.
Explicitly setting S fixes the issue.