installer: make sure all installation kernels get moved out of the way

[oliverblaha]

Newer versions of raspbian have a kernel8.img, which won't get removed when running the installation. When installing an older version than buster, e.g. stretch incorrectly boots the 64bit kernel for buster. This change makes sure that all kernel files are moved away before installing the kernel of the target version.

[kpfleming]

Please confirm that your original issue has been resolved by the patch which was included in #207.