travis: remove HAVE_RULES=yes-only build and dublicted kernel checking

If we dealing with a clang job, abort the kernel checking, so we have basically only one compiler checking the kernel with cppcheck which should be enough.

Also remove the seperate MAKEFLAGS="HAVE_RULES=yes" build, we already check that in combination with SRCDIR=yes VERIFY=1 in another job, so lets hope that is enough.

Build time on travis reduced to around 30 minutes.