main/linux-lts: require mkinitfs>=3.6.0 for compressed modules

The linux-lts and linux-virt kernels from 3.15.0 onwards use compressed kernel modules. However support for this was only added to mkinitfs in version 3.6.0.

Some users upgrading to the kernels in either Alpine Edge or 3.15 from earlier versions have had "boot hangs" afterwards. At least some of these problems appear to have occurred when "apk -aU upgrade" was not run beforehand to ensure a suitable mkinitfs was installed. This change adds an explicit minimum mkinitfs version dependancy to resolve this issue.

Fixes #13218 (closed).