use PKG_CONFIG variable for pkg-config calls
use PKG_CONFIG variable for pkg-config calls

Using a variable for pkg-config calls allows to pass arguments to
pkg-config calls which is useful for static linking.

Signed-off-by: Stefan Agner <8bf81831f67286fff829779134e88c94b37210a1@toradex.com>

boundarydevices/imx_usb_loader,bwalle/imx_usb_loader