arm: socfpga: stratix10: add CONFIG_SPL_TARGET

Stratix10 combines the u-boot-spl image into the fpga configuration
bitstream so that the SDM can load the processors memory.  This
process requires a hex format of the u-boot-spl image.
CONFIG_SPL_TARGET is set to "spl/u-boot-spl.hex"
Signed-off-by: NDalon Westergreen <dwesterg@gmail.com>

include/configs/socfpga_stratix10_socdk.h

@@ -202,6 +202,7 @@ unsigned int cm_get_l4_sys_free_clk_hz(void);
  * 0x8000_0000 ...... End of SDRAM_1 (assume 2GB)
  *
  */
+#define CONFIG_SPL_TARGET		"spl/u-boot-spl.hex"
 #define CONFIG_SPL_TEXT_BASE		CONFIG_SYS_INIT_RAM_ADDR
 #define CONFIG_SPL_MAX_SIZE		CONFIG_SYS_INIT_RAM_SIZE
 #define CONFIG_SPL_STACK		CONFIG_SYS_INIT_SP_ADDR