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/P2020RDB-PC_SDCARD_defconfig b/configs/P2020RDB-PC_SDCARD_defconfig
index ae4b758..621e5b0 100644
--- a/configs/P2020RDB-PC_SDCARD_defconfig
+++ b/configs/P2020RDB-PC_SDCARD_defconfig
@@ -39,6 +39,7 @@
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
+CONFIG_SPL_TARGET="u-boot-with-spl.bin"
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_PBSIZE=276