barebox: add custom version option

Add custom version option as used in the linux kernel.
This way we can easily specify newer and older version.
Remove the list of the older versions because is obsoleted
by the new custom version option that is more flexible.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>