GPT conf: make things easier for us

Simplify the specification of the GPT partition

The user can now specify the partitions only in the orider
from the beginning of the disk to the end,
unless he uses Index(). Doing otherwise would mean that
we would need to write a custom constraint solver and
that looks like to much work for no reason.

Clarified everything in the configuration file

Co-authored-by: name <user@users.noreply.github.com>
Signed-off-by: Luca Fulchir <luker@fenrirproject.org>