Add user-defined SR OS Bof commands

[hellt]

Currently users have no way to provide commands to the BOF file. A workaround with good-boot-exec explained in the docs for SR OS kind is not the easiest one to implement and doesn't allow using partial configs.

A proposal for vrnetlab code to read the commands from a /tftpboot/user.bof and apply them on top of the default BOF configuration. That way users may add and overwrite default bof settings.