Kconfig: Centralize prompting for SYS_CONFIG_NAME

Generally speaking, we do not prompt for this value and define it in the
board specific Kconfig file. There are some valid use cases however
today where we do prompt for this value, so instead of having this be
done in a number of locations, do this at the top-level location only.

This removes the question from a number of other locations and makes it
consistent that when we do set the value directly, we always do it the
same way. We don't need to specify the type, it's always string.

Signed-off-by: Tom Rini <trini@konsulko.com>