fix build system to correctly parse platforms

[vojtapolasek]

Description:

• read both "platform" and "platforms" while parsing yaml files
• then later work only with "platforms" but make sure that content of "platform" gets propagated into "platforms"

Rationale:

• previous introduction of multiple independent platforms in implement support for multiple platforms connected with disjunction #6661 caused a breakage in platform applicability. This is the fix.

[pep8speaks]

Hello @vojtapolasek! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-03-29 10:00:16 UTC

[openscap-ci]

Changes identified:
Others:
 Changes in Python files.

Show details

Others:
 Python abstract syntax tree change found in ssg/build_yaml.py.

Recommended tests to execute:
 (cd build && cmake ../ && ctest -j4)

[yuumasato]

What do you think about making platforms a set()?
I think it will simplify the code a bit.

[vojtapolasek]

I pushed changes which initialize platforms as set. They are also stored as set within compiled rules... I am not sure if it breaks something, it looks like not at the first glance.