rewrite efi_combo_updater using as much C as possible

[barbieri]

when I asked to not use shell, we ended with a "shell written in C", which is not good:

https://github.com/ostroproject/meta-ostro/blob/master/meta-ostro/recipes-swupd/swupd-client/files/efi_combo_updater.c

The idea is that this kind of tool can be run as a standalone binary needing as few dependencies as possible, then in a "shell-less" environment (end-goal), we do not need either busybox or toybox.

Then popen() or system() are not acceptable solutions. Almost all commands there are close to their libC alternatives, then we can replace these easily. What is left is sgdisk, but we need to figure out if can be done in pure libC without copying lots of code from sgdisk.

[barbieri]

@igor-stoppa please direct this to some future milestone and someone that should do it.

[igor-stoppa]

I'd think it safer and easier to maintain to treat gptfdisk as a library and use the sources as they are.
This will allow to rebase easily and get immediately any bugfix.

[barbieri]

what I mean is that I don't know what happens inside the sgdisk. Usually to flag a partition some value is super-simple, unlike formatting and doing more complex tasks. So we'd check what is required to get it done, if 5-simple-lines then reimplement, no need to pull in a whole project as dependency.