travis: replace kernel check with clang check.

Check clang instead of kernel, which is a lot faster.
Make clang check failure fatal to the build.

remove SRCDIR=build VERIFY=1 jobs, we already have the same just with HAVE_RULES added.
also check VERIFY=1 in combination with -DCHECK_INTERNAL ( #4798 #5869)


CURRENT ISSUES:
cppcheck throws internal errors its own code when compiled with DCHECK_INTERNAL and VERIFY=1, SRCDIR=build
cppcheck crashes on two files in the clang testsuit
cppcheck throws numerous (around 50 !!) internal errors on clang testsuite