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/m53menlo_defconfig b/configs/m53menlo_defconfig
index 455f196..2ccea83 100644
--- a/configs/m53menlo_defconfig
+++ b/configs/m53menlo_defconfig
@@ -37,6 +37,7 @@
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0x70004000
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_TARGET="u-boot-with-nand-spl.imx"
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32
 CONFIG_SYS_CBSIZE=1024