images: add core-image-initramfs-boot

images: add core-image-initramfs-boot

Add a minimal initramfs image recipe that just contains enough initramfs
to find the real root filesystem and pivot to it.

BSPs can use MACHINE_EXTRA_RRECOMMENDS to pull in any kernel module
packages that are required to bring up the hardware so that the rootfs
can be found.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>