uboot-tools: Honor the global verbose flag

Look for OPENWRT_VERBOSE and pass it down to the U-Boot so we actually
get verbose build info when needed for debugging.

Link: https://github.com/openwrt/openwrt/pull/18515
Signed-off-by: Robert Marko <robimarko@gmail.com>

package/boot/uboot-tools/Makefile

@@ -80,7 +80,8 @@ MAKE_FLAGS += \
 	ARCH="sandbox" \
 	TARGET_CFLAGS="$(TARGET_CFLAGS)" \
 	TARGET_LDFLAGS="$(TARGET_LDFLAGS)" \
-	NO_PYTHON=1
+	NO_PYTHON=1 \
+	V=$(if $(findstring c,$(OPENWRT_VERBOSE)),1,)
 
 define Build/Compile