After commit 75dd47472b "kbuild: remove src and obj from the top
Makefile", $(obj)/.kernelvariables expands to /.kernelvariables, so
our kernel variable overrides are completely ignored.
At the point in the top-level Makefile where we want to include
.kernelvariables, the current directory is guaranteed to be the object
directory, so we can simply include .kernelvariables from the current
directory.
Fixes FTBFS.