Point the tool at any workspace, don't require specific setup

[emersonknapp]

Closes #137

This change removes the requirement for a specific directory structure

• Make the target workspace a required positional argument
• Automatically create a directory, similar to the "sysroot directory" that we previously had users create manually, for the docker build context - it contains the necessary qemu binary, dockerfiles, scripts, and any other extra user inputs
• Update README.dmd to describe the simplified process

Signed-off-by: Emerson Knapp emerson.b.knapp@gmail.com

[codecov[bot]]

Codecov Report

Merging #163 into master will increase coverage by 3.7%. The diff coverage is 97.01%.

@@            Coverage Diff            @@
##           master     #163     +/-   ##
=========================================
+ Coverage   89.96%   93.66%   +3.7%     
=========================================
  Files           7        7             
  Lines         259      221     -38     
=========================================
- Hits          233      207     -26     
+ Misses         26       14     -12

Flag	Coverage Δ
#unittests	93.66% <97.01%> (+3.7%)	:arrow_up:

Impacted Files	Coverage Δ
ros_cross_compile/docker_client.py	100% <100%> (ø)	:arrow_up:
ros_cross_compile/platform.py	100% <100%> (ø)	:arrow_up:
ros_cross_compile/dependencies.py	100% <100%> (ø)	:arrow_up:
ros_cross_compile/sysroot_creator.py	100% <100%> (+2.17%)	:arrow_up:
ros_cross_compile/ros_cross_compile.py	83.33% <85.71%> (+17.87%)	:arrow_up:

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