xpl: Rename u_boot_first_phase to xpl_is_first_phase()

This is a better name for this function, so update it.

Tidy up the function comment to mention VPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/spl/spl.c b/common/spl/spl.c
index c13b2b8..9c3bba4 100644
--- a/common/spl/spl.c
+++ b/common/spl/spl.c
@@ -464,13 +464,13 @@
 		gd->malloc_ptr = 0;
 	}
 #endif
-	ret = bootstage_init(u_boot_first_phase());
+	ret = bootstage_init(xpl_is_first_phase());
 	if (ret) {
 		debug("%s: Failed to set up bootstage: ret=%d\n", __func__,
 		      ret);
 		return ret;
 	}
-	if (!u_boot_first_phase()) {
+	if (!xpl_is_first_phase()) {
 		ret = bootstage_unstash_default();
 		if (ret)
 			log_debug("Failed to unstash bootstage: ret=%d\n", ret);