Upgrade scripts for specific versions of the config file.
Each script must define a function `upgrade` that can
assume `${opt}` and `${val}` variables set on input. On
output, if `${opt}` is unset - so will be the corresponding
option.

One thing to keep in mind is that this script may be invoked
on full config - or on a defconfig. Do not assume that all
options will be present! If an option is not specified, use
the value it had as a default for the fallback; to check if
it has been specified, use `$selected_opts` associative
array to check if it has been present in the config file
(otherwise, you an option not being set after sourcing the
previous version of the config is ambiguous - it could be
that the option is called out as `# CT_FOO is not set` (i.e.
selected as false) or not present at all (i.e. selected as
default).

If it is not possible to determine the settings of an option
because a different controlling option is not set, either
err out (see CT_LIBC_UCLIBC_CUSTOM in v0 for an example)
or select a sane default and issue a warning (see CT_GDB_CUSTOM
in v0 for an example).