build: use quiet cmd for AR of libzephyr.a

Introduce a quiet_cmd to create the .a for libzephyr.a so that we can
get see explicitly what's going on if V=1 is set

Change-Id: Id0a3cf0aefac4f28bab1da3ba244b7dcead394a8
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
1 file changed