arm: socfpga: Add missing CONFIG_BUILD_TARGET

Add the missing CONFIG_BUILD_TARGET to get u-boot-with-spl.sfp built
automatically upon running make in the source tree.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Chin Liang See <clsee@altera.com>
Cc: Denis Bakhvalov <denis.bakhvalov@nokia.com>