scripts: kconfig: Fix uses of edt.compat2okay in helper functions

Check if a given 'compat' is a key in edt.compat2okay before accessing
its entry in dt_compat_on_bus() and dt_nodelabel_has_compat(), as for
a non-existing key a new entry with an empty list would be created,
and dt_compat_enabled() would then incorrectly return "y" when called
for the same 'compat'.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>