Makefile: fix usage of kernelrelease with M=foo

[nvidia]

Commit 1cb86b6c3136 ("kbuild: save overridden KERNELRELEASE in
include/config/kernel.release") set the command that is used for
printing the output of kernelrelease only when KBUILD_EXTMOD is not set.

Test:

make M=$PWD -s kernelversion
make M=$PWD -s kernelrelease

Should print both. Prints only kernelversion and an empty line for
kernelrelease. Included a simple patch that fixes this test.