checks: run qconfig sync for selected defconfigs

[ozan956]

Replace the defconfig assert step with a check_qconfig() helper
that runs qconfig.py on the selected defconfigs and reports any issues.

[ozan956]

Checks on the functionality of the workflow

Successful run:
https://github.com/analogdevicesinc/u-boot/actions/runs/23285274026/job/67707998068#step:9:34
Unsuccessful run:
https://github.com/analogdevicesinc/u-boot/actions/runs/23285608384/job/67708228214#step:9:35

[ozan956]

We decided to replace defconfig checks that is based on make defconfig with the qconfig.py, since it already includes that functionality and brings more useful checks on defconfigs.

See the test runs here:
https://github.com/analogdevicesinc/u-boot/actions/runs/23292194091/job/67731708726
https://github.com/analogdevicesinc/u-boot/actions/runs/23293850361/job/67735983551#step:10:35

[pamolloy]

@gastmaier can you take a look at this PR? You may also be interested in https://docs.u-boot.org/en/latest/develop/qconfig.html#qconfig-querying-config-options

[gastmaier]

For context, the o.g. implementation is copy from
https://github.com/analogdevicesinc/linux/blob/ci/ci/build.sh#L370-L412

[gastmaier]

So qconfig.py has a few options.
You are calling with -d a file containing a list of defconfigs to move and -s force sync by savedefconfig.
There is also a --test option for unit tests but

 """Run doctests and unit tests (so far there are no unit tests)"""

:)