fixed CFLAGS passing to Makefile

"-Wall -Werror" are CFLAGS rather than Make-flags.
this should allow to actually build :-)

additionally use KERNEL_VERSION variable, so the kernel-version can be
overridden from the cmdline using
$ make KERNEL_VERSION="6.66"
Merge pull request #2 from iem-projects/master

fixed CFLAGS passing to Makefile