cmake: yaml: switch to yaml for intermediate file

The intermediate file used to expand the generator expressions is now in
YAML format. This allows for a more robust handling of the data, as the
single quoted strings are a lot easier to escape: inside them every
character is a literal except for two single quotes (which intuitively
map to a literal single quote).

When saving a simple YAML file without genexes, the single quotes in
strings need to be escaped. However, doing so when saving the
intermediate file would make it harder to properly escape them later,
since it could be possible that the expansion of the generator
expressions would introduce new single quotes. For this reason, when
genexes are enabled, the escaping is now done in a single pass inside
the yaml-filter.cmake script.

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