cleankernel: A target to delete the kernel compile file

With the meta-build, it's always a NO_CLEAN build. Provide a way to
remove so one can rebuild from scratch. 'cleankernel' will delete the
kernel and modules object directories. Document this in build(7).

Sponsored by: Netflix
Reviewed by: debdrup, markj
Differential Revision: https://reviews.freebsd.org/D32978