With the addition of 15afd9d25 ("ARM: at91: Add initial support for the
EVB-KSZ9477 eval board"), we now have two multi-image capable boards
supported, but users still have to choose a specific SoC, so they can't
be selected at the same time.

Fix this, by restricting the menu to the non-multi-image configurations,
i.e. CONFIG_ARCH_* symbols become specific to non-multi-image.

The multi-image configurations on the other hand directly select the
CONFIG_SOC_* symbol they require. Existing CONFIG_ARCH_* usage is
adjusted to align with the new usage.

Eventually, we would want to have a at91_multi_defconfig along with
this, but for now leave existing configs as is.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>