include: Use CONFIG_XPL_BUILD instead of CONFIG_SPL_BUILD

Use the new symbol to refer to any 'SPL' build, including TPL and VPL

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/mt7621.h b/include/configs/mt7621.h
index e6dba70..636d62d 100644
--- a/include/configs/mt7621.h
+++ b/include/configs/mt7621.h
@@ -15,7 +15,7 @@
 #define CFG_SYS_INIT_SP_OFFSET		0x800000
 
 /* Serial SPL */
-#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_SERIAL)
+#if defined(CONFIG_XPL_BUILD) && defined(CONFIG_SPL_SERIAL)
 #define CFG_SYS_NS16550_CLK		50000000
 #define CFG_SYS_NS16550_COM1		0xbe000c00
 #endif