sdk: update SDK Makefile

the SDK's Makefile referenced the old file name.
Update it too.

Fixes: 2d5f7035cf45 ("sdk: rename README")
Signed-off-by: Christian Lamparter <[email protected]>

target/sdk/Makefile

@@ -145,7 +145,7 @@ $(BIN_DIR)/$(SDK_NAME).tar.xz: clean
 		./files/Config.in \
 		./files/Makefile \
 		./files/include/prepare.mk \
-		./files/README.SDK \
+		./files/README.md \
 		$(SDK_BUILD_DIR)/
 	mkdir -p $(SDK_BUILD_DIR)/package/kernel
 	$(CP) \