Add max_size option to ConsoleExpectMixin read

[marc-roeoesli]

Extend read function of serialdriver and externalconsoledriver with 'max_size' argument that limits the number of read bytes to 'max_size'. Set 'size' = 'max_size' to read exactly 'size' bytes. The values 'None' and '0' result in an unlimited read.
Extend 'test_read' in test_serialdriver.py and test_externalconsoledriver.py to check 'max_size' argument.
Implement 'max_size' explicitely in consoleexpectmixin.

• We use the feature to read a fixed number of bytes on the serial port during testing the serial communication. The current read function returns the full buffer of the serial port (maximum of size and in_waiting). We use labgrid to test embedded software on hardware devices.
• We think that for certain tests it can be useful to read a well defined and fixed size from the serial port. We hope that this could also be useful for other developers. We are open to discuss other possibilities/suggestions.
• We tested the feature by extending the existing test for the serial driver and external console driver 'read'-function.

Checklist

• Documentation for the feature
• Tests for the feature
• Add a section on how to use the feature to doc/usage.rst
• CHANGES.rst has been updated
• PR has been tested
• Man pages have been regenerated

Signed-off-by: Marc Roeoesli marc.roeoesli@mt.com

[codecov]

Codecov Report

Merging #912 (b4504ca) into master (1d1cecb) will increase coverage by 0.0%.
The diff coverage is 100.0%.

❗ Current head b4504ca differs from pull request most recent head f5226ed. Consider uploading reports for the commit f5226ed to get more accurate results

@@          Coverage Diff           @@
##           master    #912   +/-   ##
======================================
  Coverage    56.8%   56.8%           
======================================
  Files         149     149           
  Lines       11091   11096    +5     
======================================
+ Hits         6301    6306    +5     
  Misses       4790    4790           

Impacted Files	Coverage Δ
labgrid/driver/consoleexpectmixin.py	74.0% <100.0%> (+0.9%)	⬆️
labgrid/driver/externalconsoledriver.py	82.3% <100.0%> (+0.8%)	⬆️
labgrid/driver/serialdriver.py	81.4% <100.0%> (+0.4%)	⬆️
labgrid/remote/config.py	90.0% <0.0%> (-0.5%)	⬇️
labgrid/remote/exporter.py	60.4% <0.0%> (-0.1%)	⬇️

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[Emantor]

IMO the RFC from the commit message can be removed.

[jluebbe]

I've squashed the commits together and made minor changes to the message. I've also removed the RFC tag.