devtool/upgrade: check all git config locations

devtool/upgrade: check all git config locations

Fixes [YOCTO #15222]

In some situations its more practical to keep git configuration
at `/etc/gitconfig` instead of `$HOME/.gitconfig` (e.g., when mounting
git configuration into a docker container).

This change makes `devtool upgrade` consider any available
git configuration instead of only checking `--global`.

Signed-off-by: Marcus Flyckt <marcus.flyckt@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>