MEDIUM: initcall: allow to register mutiple post_section_parser per section

MEDIUM: initcall: allow to register mutiple post_section_parser per section

Before this patch, REGISTER_CONFIG_SECTION() allowed to register one and only
one callback (<post>) called after the parsing of a section.

It was limitating because you couldn't register a post callback from anywhere
else in the code.

This patch introduces the new REGISTER_CONFIG_SECTION_POST() macros which allows
to register a new post callback for a section keyword from anywhere.

This patch introduces the feature by allowing `struct cfg_section` entries that
does not have a `section_parser`, and then iterating on all cfg_section with a
post_section_parser for a keyword.