testbuild:Added -x to fail build on errors for CI

   On CI we want to know ASAP of a failure. This adds the
   -x (exit on build failures) option to faclitate that
   behavior.

   Use ${MAKE} ${MAKE_FLAGS} for make invocation.
   When -x is provided change MAKE_FLAGS to
   --silent --no-print-directory and set -e

   Ignore exit status when using grep for checking
   for CONFIG_NXWM=y