Fix requires handling for baremetal config

[rwalton-arm]

Description

We were previously replacing the list of "requires" if we happened to find a set of requires not in the app config. This meant occasionally some config macros would not be defined for baremetal apps depending on the order the config files were discovered and parsed.

Now we always append to the list of requires, which is how it's supposed to work.

Test Coverage

• [x] This change is covered by existing or additional automated tests.
• [ ] Manual testing has been performed (and evidence provided) as automated testing was not feasible.
• [ ] Additional tests are not required for this change (e.g. documentation update).

[codecov[bot]]

Codecov Report

Merging #303 (0906ceb) into master (580ff28) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #303      +/-   ##
==========================================
- Coverage   97.08%   97.08%   -0.01%     
==========================================
  Files          92       92              
  Lines        2782     2776       -6     
==========================================
- Hits         2701     2695       -6     
  Misses         81       81              

Impacted Files	Coverage Δ
src/mbed_tools/build/config.py	100.00% <ø> (ø)
...ls/build/_internal/config/assemble_build_config.py	100.00% <100.00%> (ø)
src/mbed_tools/build/_internal/config/config.py	100.00% <100.00%> (ø)
src/mbed_tools/build/_internal/config/source.py	100.00% <100.00%> (ø)