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/ls2080aqds_nand_defconfig b/configs/ls2080aqds_nand_defconfig
index 209202d..dab9a7f 100644
--- a/configs/ls2080aqds_nand_defconfig
+++ b/configs/ls2080aqds_nand_defconfig
@@ -45,6 +45,7 @@
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_TARGET="u-boot-with-spl.bin"
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_GREPENV=y