os-util: add a new confext image type and the ability to parse their release files

os-util: add a new confext image type and the ability to parse their release files

Adds a new image type called IMAGE_CONFEXT which is similar to IMAGE_SYSEXT but works
for the /etc/ directory instead of /usr/ and /opt/. This commit also adds the ability to
parse the release file that is present with the confext image in /etc/confext-release.d/
directory.