kernel: remove .config.prev when running make kernel_*config to fix rebuild errors

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 48545

include/kernel-build.mk

@@ -125,6 +125,7 @@ define BuildKernel
 	$(MAKE) -C image compile TARGET_BUILD=
 
   oldconfig menuconfig nconfig: $(STAMP_PREPARED) $(STAMP_CHECKED) FORCE
+	rm -f $(LINUX_DIR)/.config.prev
 	rm -f $(STAMP_CONFIGURED)
 	$(LINUX_RECONF_CMD) > $(LINUX_DIR)/.config
 	$(_SINGLE)$(MAKE) -C $(LINUX_DIR) $(KERNEL_MAKEOPTS) $$@