tbs2910: Disable Plan9/RTEMS bootm support

We have once again reached a point where this board does not build in
some cases with supported toolchains due to reaching a size constraint.
To regain some space, disable support for Plan 9 / RTEMS images with the
bootm command.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stefano Babic <sbabic@denx.de>