fips: make FIPS140-2 mode enablement logic simpler

Previously, to enable the FIPS140-2 mode, both /etc/system-fips and the fips=1 kernel command line need to be set. While this was designed to be consistent, the convention is not well followed by the other crypto libraries and the former tends to be ignored. This aligns the behavior to the latter, i.e. if fips=1 is set, the library enables the FIPS140-2 mode regardless of the existence of /etc/system-fips.

Suggested by Alexander Sosedkin.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code