Convert CONFIG_SYS_SPL_ARGS_ADDR to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_SPL_ARGS_ADDR

In doing so, we also consistently use this variable for SPL_OS_BOOT and
not CONFIG_SYS_FDT_BASE in some cases.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/spl_nor.c b/common/spl/spl_nor.c
index 067a2d4..7986e93 100644
--- a/common/spl/spl_nor.c
+++ b/common/spl/spl_nor.c
@@ -74,8 +74,8 @@
 			       (void *)(CONFIG_SYS_OS_BASE +
 					sizeof(struct image_header)),
 			       spl_image->size);
-#ifdef CONFIG_SYS_FDT_BASE
-			spl_image->arg = (void *)CONFIG_SYS_FDT_BASE;
+#ifdef CONFIG_SYS_SPL_ARGS_ADDR
+			spl_image->arg = (void *)CONFIG_SYS_SPL_ARGS_ADDR;
 #endif
 
 			return 0;