configs: Enable EXT4 and ISO partitions for the DeveloperBox

Since this box is SystemReady compliant enable ISO_PARTITION which is
needed to start some installers (e.g Fedora).  While at it enable EXT4
as well which is a common filesystem for targets

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>