Convert CONFIG_SPL_INIT_MINIMAL et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_INIT_MINIMAL
   CONFIG_SPL_FLUSH_IMAGE
   CONFIG_SPL_SKIP_RELOCATE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/powerpc/cpu/mpc85xx/start.S b/arch/powerpc/cpu/mpc85xx/start.S
index 2b2ad97..7a079ed 100644
--- a/arch/powerpc/cpu/mpc85xx/start.S
+++ b/arch/powerpc/cpu/mpc85xx/start.S
@@ -27,7 +27,7 @@
 #define LAW_EN		0x80000000
 
 #if defined(CONFIG_NAND_SPL) || \
-	(defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_INIT_MINIMAL))
+	(defined(CONFIG_SPL_BUILD) && CONFIG_IS_ENABLED(INIT_MINIMAL))
 #define MINIMAL_SPL
 #endif