u-boot.mk: fix pkg-config usage

Using Host/Exports doesn't work as intended, explicitly add the
required vars so that u-boot finds the required libraries when building
its tools.

Signed-off-by: Andre Heider <a.heider@gmail.com>

include/u-boot.mk

@@ -45,6 +45,9 @@ UBOOT_MAKE_FLAGS = \
 	HOSTCC="$(HOSTCC)" \
 	HOSTCFLAGS="$(HOST_CFLAGS) $(HOST_CPPFLAGS) -std=gnu11" \
 	HOSTLDFLAGS="$(HOST_LDFLAGS)" \
+	STAGING_PREFIX="$(STAGING_DIR_HOST)" \
+	PKG_CONFIG_PATH="$(STAGING_DIR_HOST)/lib/pkgconfig" \
+	PKG_CONFIG_LIBDIR="$(STAGING_DIR_HOST)/lib/pkgconfig" \
 	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='')
 
 define Build/U-Boot/Target