sysbuild: Add support for snippets

Adds support for sysbuild loading snippets, these can be included
by using e.g.: cmake ... -DSB_SNIPPET=blah for sysbuild
directly or can be used with an application and sysbuild using
-DSNIPPET. Snippets for sysbuild can use SB_EXTRA_CONF_FILE in the
snippet file to specify an extra Kconfig fragment for sysbuild

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>