config: support new symbol intro'd in kernel 4.12

Symbol CONFIG_INITRAMFS_FORCE allows to ignore the value passed by the
bootloader.

By default, all symbols containing INITRAMFS are wiped from the final
config and then re-added conditionally.

Add support for this symbol, as the build will stop otherwise
questioning the user about this option:

* Restart config...
*
*
* General setup
*
Cross-compiler tool prefix (CROSS_COMPILE) []
Compile also drivers which will not load (COMPILE_TEST) [N/y/?] n

...

Initial RAM filesystem and RAM disk (initramfs/initrd) support
(BLK_DEV_INITRD) [Y/n/?] y
Initramfs source file(s) (INITRAMFS_SOURCE) []
Ignore the initramfs passed by the bootloader (INITRAMFS_FORCE)
[N/y/?] (NEW)

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

config/Config-images.in

@@ -52,6 +52,13 @@ menu "Target Images"
 			help
 			  Kernel uses specified external cpio as INITRAMFS_SOURCE.
 
+		config TARGET_INITRAMFS_FORCE
+                        bool "Force"
+                        depends on TARGET_ROOTFS_INITRAMFS
+                        default n
+                        help
+                          Ignore the initramfs passed by the bootloader.
+
 	comment "Root filesystem archives"
 
 	config TARGET_ROOTFS_CPIOGZ

include/kernel-defaults.mk

@@ -63,6 +63,8 @@ ifeq ($(CONFIG_TARGET_ROOTFS_INITRAMFS),y)
 	$(call Kernel/SetInitramfs/PreConfigure)
 	echo 'CONFIG_INITRAMFS_ROOT_UID=$(shell id -u)' >> $(LINUX_DIR)/.config
 	echo 'CONFIG_INITRAMFS_ROOT_GID=$(shell id -g)' >> $(LINUX_DIR)/.config
+	echo 'CONFIG_INITRAMFS_FORCE=y' >> $(LINUX_DIR)/.config
+	echo "$(if $(CONFIG_TARGET_INITRAMFS_FORCE),CONFIG_INITRAMFS_FORCE=y,# CONFIG_INITRAMFS_FORCE is not set)" >> $(LINUX_DIR)/.config
 	echo "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_NONE),CONFIG_INITRAMFS_COMPRESSION_NONE=y,# CONFIG_INITRAMFS_COMPRESSION_NONE is not set)" >> $(LINUX_DIR)/.config
 	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_GZIP),CONFIG_INITRAMFS_COMPRESSION_GZIP=y\nCONFIG_RD_GZIP=y,# CONFIG_INITRAMFS_COMPRESSION_GZIP is not set\n# CONFIG_RD_GZIP is not set)" >> $(LINUX_DIR)/.config
 	echo -e "$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_BZIP2),CONFIG_INITRAMFS_COMPRESSION_BZIP2=y\nCONFIG_RD_BZIP2=y,# CONFIG_INITRAMFS_COMPRESSION_BZIP2 is not set\n# CONFIG_RD_BZIP2 is not set)" >> $(LINUX_DIR)/.config
@@ -78,6 +80,7 @@ define Kernel/SetNoInitramfs
 	mv $(LINUX_DIR)/.config.set $(LINUX_DIR)/.config.old
 	grep -v INITRAMFS $(LINUX_DIR)/.config.old > $(LINUX_DIR)/.config.set
 	echo 'CONFIG_INITRAMFS_SOURCE=""' >> $(LINUX_DIR)/.config.set
+	echo '# CONFIG_INITRAMFS_FORCE is not set' >> $(LINUX_DIR)/.config.set
 endef
 
 define Kernel/Configure/Default