config: support new symbol intro'd in kernel 4.12

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>