implement a more reliable /boot detection - specially for partitionless systems

[dorileo]

When mounting boot (on bootman.c) we try to detect if /boot is mounted, also we need to consider "partitionless" /boot systems. For that right now we're checking if /boot is empty, we need a more reliable strategy.

Quoting @bryteise here:

I think this will work in most cases for extlinux as that writes a file that can't really
be removed without some hackery to the filesystem. I do think we likely need to
improve detection for repair scenarios where that file may not exist but it is a low
priority.