makefile: silent help target

Instead of prefixing the 'do not echo command' '@', make help as
prerequisite of .SILENT. This avoids prefixing '@' in future help
messages.

Change-Id: If33b82b4d3bccc92e8de74703ff6af04387008a0
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
1 file changed