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/ls1088aqds_sdcard_qspi_defconfig b/configs/ls1088aqds_sdcard_qspi_defconfig
index 40306d6..f3e2048 100644
--- a/configs/ls1088aqds_sdcard_qspi_defconfig
+++ b/configs/ls1088aqds_sdcard_qspi_defconfig
@@ -54,6 +54,7 @@
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
+CONFIG_SPL_TARGET="u-boot-with-spl.bin"
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_GREPENV=y