Pull Platform out of SysrootCreator for reuse

[emersonknapp]

• DockerConfig and Platform had close knowledge of each other, consolidate their responsibility
• Moving Platform out into a separate file so that it can be used by other upcoming steps of the cross-compile pipeline.
• Removed the dangling unused field cc_toolchain

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

[codecov[bot]]

Codecov Report

Merging #138 into master will increase coverage by 1.4%. The diff coverage is 92.85%.

@@            Coverage Diff            @@
##           master     #138     +/-   ##
=========================================
+ Coverage   82.15%   83.56%   +1.4%     
=========================================
  Files           4        5      +1     
  Lines         213      213             
=========================================
+ Hits          175      178      +3     
+ Misses         38       35      -3

Flag	Coverage Δ
#unittests	83.56% <92.85%> (+1.4%)	:arrow_up:

Impacted Files	Coverage Δ
ros_cross_compile/platform.py	100% <100%> (ø)
ros_cross_compile/sysroot_creator.py	86.6% <40%> (-2.71%)	:arrow_down:
ros_cross_compile/ros_cross_compile.py	68.29% <80%> (+3.29%)	:arrow_up:

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[emersonknapp]

I made the types immutable via tuple instead of frozenset