Makefile.include: revised verbosity control

This should be more friendly to legacy operating systems that
don't support multiple shell commands per line. Note that
architecture-specific overrides need to be adapted, if verbosity
control is desired for them as well.
Merge pull request #309 from frtos-wpan/verbosity-v2

Makefile.include: revised verbosity control
Be consistent with tab chars in Makefiles - yet again...