Introduce MODULES_EXCLUDE

It's sometimes easier to exclude some modules rather than listing all
possibly needed ones with MODULES_OVERRIDE.
So for this add MODULES_EXCLUDE which do exactly as one would guess, excludes
some modules from the build/install.

For example if one wants to exclude all modules which are only present in the
GENERIC config on amd64 :
export MODULES_EXCLUDE=$(grep -E '^device' sys/amd64/conf/GENERIC | awk '{print $2}' | tr '\n' ' ')

MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33443