build: suppress cmake warnings about unused vars

build: suppress cmake warnings about unused vars

When cmake is invoked to build a package it usually reports a warning
about unused variables passed to it. This is caused by openwrt passing
all supported variables to cmake, even if they are not all required by
the package being compiled.

To reduce clutter when compiling such packages these warnings are now
suppressed.

Approved-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Tjalling Hattink <t.hattink@fugro.com>