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/ls1046ardb_qspi_spl_defconfig b/configs/ls1046ardb_qspi_spl_defconfig
index c081b78..2f6b546 100644
--- a/configs/ls1046ardb_qspi_spl_defconfig
+++ b/configs/ls1046ardb_qspi_spl_defconfig
@@ -58,6 +58,7 @@
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SYS_OS_BASE=0x40980000
 CONFIG_SPL_WATCHDOG=y
+CONFIG_SPL_TARGET="spl/u-boot-spl.pbl"
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_SPL=y