part: Disable CONFIG_SPL_ISO_PARTITION by default

We enabled CONFIG_ISO_PARTITION by default for distro boot, so that U-Boot
could load distro images that usually get shipped as iso images. These images
usually come with a board agnostic boot environment.

However, there is very little point in having ISO support enabled (for anyone
really) in SPL, as the whole idea of SPL is to load U-Boot proper which again
is board specific. So the fact that we enable ISO support in U-Boot proper does
not mean at all that we want ISO support in U-Boot SPL.

Hence, let's remove the Kconfig dependency. Along the way, let's also clean up
all those default configs that disabled SPL ISO support.

Signed-off-by: Alexander Graf <agraf@suse.de>