scripts: footprint: Fix size_report on freestanding apps

os.path.commonpath() throws a ValueError exception when paths in the
list provided share nothing in common (like for example two Windows
paths on different drive letters). Handle that special case properly
instead of letting the exception propagate up.

Fixes #51549.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>