sysbuild: Support SB_CONF_FILE and SB_EXTRA_CONF_FILE as lists

These variables need to be converted to absolute paths internally, but
so far this has only worked when each value consisted of a single path.
Add a common loop to handle lists of paths.

As a bonus, run `string(CONFIGURE)` to expand those variables in the
same way as the regular CONF_FILE and EXTRA_CONF_FILE.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>