mx23: config: Enable building of u-boot.sb binary

For i.MX23 we need to pass imx23 as elftosb param.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>