spl, common, serial: build SPL without serial support

This patch enables building SPL without
CONFIG_SPL_SERIAL_SUPPORT support.

Signed-off-by: Heiko Schocher <hs@denx.de>
[trini: Ensure we build arch/arm/imx-common on mx28]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/common/cli_readline.c b/common/cli_readline.c
index 9a9fb35..c1476e4 100644
--- a/common/cli_readline.c
+++ b/common/cli_readline.c
@@ -597,7 +597,7 @@
 					puts(tab_seq + (col & 07));
 					col += 8 - (col & 07);
 				} else {
-					char buf[2];
+					char __maybe_unused buf[2];
 
 					/*
 					 * Echo input using puts() to force an