Add support for mounting /boot

[paulmenzel]

[paulmenzel]

This still needs to be tested, and should be considered as an RFC.

[paulmenzel]

Creating the FS on the block device, and handling, if it’s a directory, is still missing.

[mika]

@paulmenzel ping, are you still interested in getting this submitted? :)

[jkirk]

I stumbled over this today, so it would be really nice if we can get this into grml-debootstrap. :)