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/P1020RDB-PC_36BIT_NAND_defconfig b/configs/P1020RDB-PC_36BIT_NAND_defconfig
index 3e5c044..80bfe31 100644
--- a/configs/P1020RDB-PC_36BIT_NAND_defconfig
+++ b/configs/P1020RDB-PC_36BIT_NAND_defconfig
@@ -43,6 +43,7 @@
 CONFIG_TPL_RELOC_MALLOC_ADDR=0xf8fb4000
 CONFIG_TPL_RELOC_MALLOC_SIZE=0xc000
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_TARGET="u-boot-with-spl.bin"
 CONFIG_TPL=y
 CONFIG_TPL_MAX_SIZE=0x20000
 CONFIG_TPL_ENV_SUPPORT=y