cmake: support generator expressions in build_info()

This patch enables support for genexes in the 'build_info()' function by
identifying when arguments contain them and passing the GENEX flag to
'yaml_set()'.

Genexes are not supported in tag lists; using them will immediately
trigger an error by the schema checks.

They are also not currently supported with PATH entries, because the
actual contents of the list (whose paths are to be converted) are not
known until after the CMake code has been processed.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>