INITRD var: make it a list of filesystem images

INITRD var: make it a list of filesystem images

The initrd image used by the Linux kernel is list of file system images
concatenated together and presented as a single initrd file at boot time.

So far the initrd is a single filesystem image. But in cases like to support
early microcode loading, the initrd image need to have multiple filesystem
images concatenated together.

This commit is extending the INITRD variable from a single filesystem image
to a list of filesystem images to satisfy the need mentioned above.

(From OE-Core rev: b0ac481dda99d8f4be8015964fcb2cb01afce08c)

Signed-off-by: Nitin A Kamble <nitin.a.kamble@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>