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/P1010RDB-PA_NAND_defconfig b/configs/P1010RDB-PA_NAND_defconfig
index 918a107..5813c75 100644
--- a/configs/P1010RDB-PA_NAND_defconfig
+++ b/configs/P1010RDB-PA_NAND_defconfig
@@ -41,6 +41,7 @@
 CONFIG_TPL_RELOC_MALLOC_ADDR=0xd0034000
 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_DRIVERS_MISC=y