kbuild: introduce $(kecho) convenience echo

There is a bunch of places in the build system where we do 'echo' to show
some nice status lines.  This means we still get output when running in
silent mode.  So declare a new KECHO variable that only does 'echo' when we
are in a suitable verbose build mode.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
[sam: added Documentation]
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>