RFC: Add virtual initramfs-generator package

Presently, many Alpine package depend directly on mkinitfs (e.g. grub, syslinux, or linux-lts itself). However, these package do not specifically need mkinitfs but simply a tool to generate an initramfs image. By providing a virtual initramfs-generator package we can abstract from the specifics of a initramfs generation tool and allow usage of Alpine with initramfs images not generated by mkinitfs. The initramfs generation tool simply needs to provide /boot/initramfs-$flavour.

I primarily interested in this to improve support for alternative initramfs. Most importantly, testing/booster.