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-PD_NAND_defconfig b/configs/P1020RDB-PD_NAND_defconfig
index bd3d7a3..d770fc7 100644
--- a/configs/P1020RDB-PD_NAND_defconfig
+++ b/configs/P1020RDB-PD_NAND_defconfig
@@ -42,6 +42,7 @@
 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