Barebox/UBoot/SmallUBootDriver: handle hex arguements properly

[pmelange]

Checks to see if the interrupt string starts with \x and when that is the case, converts the remained of the string to a bytearray before sending the interrupt to the boot loader.

fixes #1622

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Project coverage is 55.7%. Comparing base (2ce9e24) to head (315a968).

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/smallubootdriver.py	0.0%	8 Missing :warning:
labgrid/driver/bareboxdriver.py	0.0%	7 Missing :warning:
labgrid/driver/ubootdriver.py	0.0%	7 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1623     +/-   ##
========================================
- Coverage    55.8%   55.7%   -0.1%     
========================================
  Files         170     170             
  Lines       13382   13400     +18     
========================================
  Hits         7469    7469             
- Misses       5913    5931     +18     

Flag	Coverage Δ
3.10	55.7% <0.0%> (-0.1%)	:arrow_down:
3.11	55.7% <0.0%> (-0.1%)	:arrow_down:
3.12	55.7% <0.0%> (-0.1%)	:arrow_down:
3.13	55.7% <0.0%> (-0.1%)	:arrow_down:
3.9	55.7% <0.0%> (-0.1%)	:arrow_down:

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[aparcar]

A bit DRY, but I need this, too