omap3: sniper: Streamline defconfig

omap3: sniper: Streamline defconfig

Remove custom config options that are not particularly necessary.
Align them with OMAP3 defaults used on other boards (especially for
memory locations).

Also enable Thumb build to reduce the SPL size and remove the custom
prompt text.

This makes the config a lot more minimalistic, maintainable and easier
to read.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Derald D. Woods <woods.technical@gmail.com>