kernel: configure the kernel again after an initramfs build

Kernel/SetInitramfs writes the initramfs configuration into the kernel
tree and deletes .config.prev, so that the next configure stage puts the
regular configuration back. The check that decides if the configure stage
must run ignored this signal. The next build then compiled the regular
kernel with the initramfs configuration, and the squashfs images got a
kernel with an embedded root filesystem.

Run the configure stage if .config.prev is missing.

Fixes: e97e3f3 ("kernel: configure the kernel only when an input changed")
Signed-off-by: John Crispin <john@phrozen.org>

Author: blogic

include/kernel-build.mk

@@ -157,6 +157,7 @@ define BuildKernel
   $(KERNEL_CONFIG_CHECK): $(STAMP_PREPARED) $(LINUX_KCONFIG_LIST) $(TOPDIR)/.config FORCE
 	@mkdir -p $(LINUX_DIR)
 	@[ -f $(STAMP_CONFIGURED) ] && [ -d $(LINUX_DIR)/user_headers ] && \
+		[ -f $(LINUX_DIR)/.config.prev ] && \
 		[ -z "$$$$(find $(KERNEL_CONFIG_DEPENDS) ! -type d \
 			-newer $(STAMP_CONFIGURED) -print -quit 2>/dev/null)" ] || \
 		touch $$@