meta-intel: Ensure we pass the microcode as initrd in wic images

We need to ensure that the microcode is passed to the kernel as an
initrd to allow for application of required updates.

Bug-AGL: SPEC-1173
Change-Id: I37a2ca401df9a9c17cf2d3a60151af639a3aba28
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/12453
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>