udev-extraconf: let automount base directory configurable

udev-extraconf: let automount base directory configurable

Dont hard-code automount base directory to '/run/media', introduce a
variable MOUNT_BASE to let it configurable, like in udisks2 the mount
base is also configurable by setting option: --enable-fhs-media.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>