Add storage kernel modules for arm64/armhf build

This bug report mentions pc-kernel, but that snap is specific to amd64, so what is its relationship to this PR for arm support?

Why is this a change to be made to the initramfs support, rather than having the kernel modules required for storage access being built into the kernel image?

Steve, the pc-kernel snap is also published for armhf and arm64 (though it appears the arm64 was never published to 18/stable, only 18/candidate).

Though it's unclear if this request for UC16 or UC18?

The pc-kernel snap is essentially just "linux-generic", sadly it was decided back then that the "linux-generic" name was not appropriate and that we would only build the kernel snap for amd64 in core16 ....

In core 18 it was finally accepted to also build this kernel for both arm arches so that we can support all boards that are also supported by the linux-generic deb version of the respective arch (there are cubox, sabrelite, beaglebone, pocketbeagle and a few other community images based on the armhf pc-kernel snap around already).

Thus this request is indeed only for core18.

Yes this is for Core 18, if pc-kernel in Core 20 is not leverage this package we should have these as kernel built-in.

And yes, we are keen to build UC images with pc-kernel rather than custom kernel branches for community boards