Convert CONFIG_SPL_TARGET to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_TARGET

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/socfpga_n5x_defconfig b/configs/socfpga_n5x_defconfig
index f0d41d1..92d6045 100644
--- a/configs/socfpga_n5x_defconfig
+++ b/configs/socfpga_n5x_defconfig
@@ -34,6 +34,7 @@
 CONFIG_SPL_CACHE=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x3c00000
+CONFIG_SPL_TARGET="spl/u-boot-spl-dtb.hex"
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="SOCFPGA_N5X # "
 CONFIG_SYS_MAXARGS=64