New —-list-separator argument of confgen.py and
prepare_kconfig_files.py is used to select which character is used
as list separator. For compatibility with esp-docs, we still keep
support for space separator. Otherwise esp-docs would have to choose
the separator depending on the IDF version.
Write COMPONENT_KCONFIGS_SOURCE_FILE and
COMPONENT_KCONFIGS_PROJBUILD_SOURCE_FILE files always even when
COMPONENT_KCONFIGS or COMPONENT_KCONFIGS_PROJBUILD are empty variables
because kconfiglib expects them to exist.
