spl: Add option SPL_PAYLOAD

Some legacy boards use RAW image for SPL boot. Add Kconfig option
SPL_PAYLOAD to set alternative image.

Signed-off-by: York Sun <york.sun@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/Makefile b/Makefile
index 79cbad2..36c145b 100644
--- a/Makefile
+++ b/Makefile
@@ -1111,8 +1111,8 @@
 u-boot.dis:	u-boot
 		$(OBJDUMP) -d $< > $@
 
-ifdef CONFIG_TPL
-SPL_PAYLOAD := tpl/u-boot-with-tpl.bin
+ifneq ($(CONFIG_SPL_PAYLOAD),)
+SPL_PAYLOAD := $(CONFIG_SPL_PAYLOAD:"%"=%)
 else
 SPL_PAYLOAD := u-boot.bin
 endif