Depend on initramfs instead of mkinitcpio

[clarfonthey]

This removes the mkinitcpio dependency from most kernel packages and replaces it with initramfs to allow alternative initcpio creation, which is done on mainline Arch.

Small note: these depend on mkinitcpio>=0.7, but mkinitcpio only started providing initramfs after this version, so, this replacement is valid.

The following kernel packages were excluded from this change because they provide custom hooks that run mkinitcpio directly:

• linux-am33x
• linux-armv7-rc
• linux-espressobin
• linux-odroid-c2
• linux-odroid-n2

These hooks are probably unnecessary because mkinticpio provides its own hook, but I didn't want to make any invasive changes.

[clarfonthey]

Note: I am more than happy to rebase this once someone reviews it, but I'm not going to constantly rebase it until then. I assume that all these kernel package updates are automated.