UBootDriver: Implement Idle Console Handling

[Bastian-Krause]

Description Activating the UBootDriver for a U-Boot that is already interrupted does not work at the moment, because nothing is received on the console since nothing is sent in the first place. Additionally, a boot expression is expected during activation.

To change that, harmonize its _await_prompt() method with the one found in BareboxDriver. BareboxDriver already implements handling the idle console functionality. Also deprecate the boot_expression attribute altogether. A similar check happened to be in the BareboxDriver, which was dropped in a refactoring in #259.

This allows a strategy to force() a U-Boot state.

Checklist

• [ ] Documentation for the feature
• [ ] Tests for the feature
• [x] The arguments and description in doc/configuration.rst have been updated
• [x] CHANGES.rst has been updated
• [x] PR has been tested

[codecov[bot]]

Codecov Report

Merging #965 (eccd752) into master (d0fc794) will decrease coverage by 0.0%. The diff coverage is 44.4%.

@@           Coverage Diff            @@
##           master    #965     +/-   ##
========================================
- Coverage    57.1%   57.1%   -0.1%     
========================================
  Files         150     150             
  Lines       11203   11215     +12     
========================================
+ Hits         6402    6405      +3     
- Misses       4801    4810      +9     

Impacted Files	Coverage Δ
labgrid/driver/ubootdriver.py	57.6% <41.1%> (-3.4%)	:arrow_down:
labgrid/driver/smallubootdriver.py	45.2% <100.0%> (+1.3%)	:arrow_up:
labgrid/pytestplugin/reporter.py	86.2% <0.0%> (-0.8%)	:arrow_down:

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