Kconfig: Set default BUILD_TARGET for ARCH_MX6 with SPL

Use BUILD_TARGET to automatically build "u-boot-with-spl.imx" on MX6
targets with SPL.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>