Refactor pre-commit CI

Currently our test mechanism is not fit for building without
cross-toolchain as we are not allowing EXTRA_MAKE_FLAGS. Also we are not
allowing to build and test other KERNCONF. So allow these variables to
be overridden in command line.

Approved by: lwhsu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50220