What about overwriting it automatically when it was not modified
by the user? This would only need a backup copy of the original
default configuration. I assume that most users don't change
config-devices.mak, so they would be happy with this solution.
Calling "make defconfig" is only the second best solution for them.
A more sophisticated solution could try to create a patch from
original default configuration and user modified configuration.
Make could try to apply this patch to the new default configuration,
and if there is no conflict, all is fine as well.