sdk: keep the linux package in the same subdir as in the main build system

Fixes: 953435795d17 ("build: always build package/kernel/linux")
Signed-off-by: Felix Fietkau <[email protected]>

target/sdk/Makefile

@@ -129,11 +129,14 @@ $(BIN_DIR)/$(SDK_NAME).tar.xz: clean
 		./files/include/prepare.mk \
 		./files/README.SDK \
 		$(SDK_BUILD_DIR)/
+	mkdir -p $(SDK_BUILD_DIR)/package/kernel
 	$(CP) \
 		$(TOPDIR)/package/Makefile \
 		$(TOPDIR)/package/libs/toolchain \
-		$(TOPDIR)/package/kernel/linux \
 		$(SDK_BUILD_DIR)/package/
+	$(CP) \
+		$(TOPDIR)/package/kernel/linux \
+		$(SDK_BUILD_DIR)/package/kernel/
 
 	-rm -rf $(SDK_BUILD_DIR)/$(STAGING_SUBDIR_HOST)/.prereq-build