lv_assemble should support conditional assembly (preprocessor?)

[marcpage]

Is your feature request related to a problem? Please describe.
If I were to create .mnu files from text files, it would be handy to be able to share common code and have some way of making parts conditional based on flags passed in when assembling.

Describe the solution you'd like
Have yaml comments with conditional statements. These would include or exclude based on the conditional.

Describe alternatives you've considered
Use a C-preprocessor.

Additional context
None