Correct SPL use of CMD_PSTORE

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_CMD_PSTORE defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/image-fdt.c b/boot/image-fdt.c
index 408abce..714d05d 100644
--- a/boot/image-fdt.c
+++ b/boot/image-fdt.c
@@ -638,7 +638,7 @@
 
 	/* Update ethernet nodes */
 	fdt_fixup_ethernet(blob);
-#if CONFIG_IS_ENABLED(CMD_PSTORE)
+#if IS_ENABLED(CONFIG_CMD_PSTORE)
 	/* Append PStore configuration */
 	fdt_fixup_pstore(blob);
 #endif