pxe: Splitting generic code in init.common

init is becoming a very long file that does prepare directories, virtual
fs, detect & probe pci device and setup the network.

All this parts will be shared with some other targets that need to be
booted live like the future health_check role.

All this parts of code got moved to init.common which
- allow to share code between live booted roles
- ease the reading of init

Init is now much more compact & easier to understand to new comers.