cmake: yaml: (cosmetic) avoid referring to file type in variable names

To move away from the JSON/YAML terminology, rename the variables that
refer to the intermediate files to "expanded" and "output" files.

Also, clean up a few "C-style" function calls and a typo in a variable
name in the same files.

No functional change introduced in this commit.

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