sh: share the correct version of start.S among all cpus

It is easy to note that SH2/SH3/SH4 start.S code is practically
the same with a minor difference for SH2 where a short data header is
present. To avoid unwanted code duplication and to automatically
convert SH2 and SH3 platforms to generic board support move fixed SH4
start.S into arch/sh/lib/start.S and share it among all platforms.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sh/lib/Makefile b/arch/sh/lib/Makefile
index 7be20b1..473cf0d 100644
--- a/arch/sh/lib/Makefile
+++ b/arch/sh/lib/Makefile
@@ -5,6 +5,8 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+extra-y	+= start.o
+
 obj-y	+= board.o
 obj-$(CONFIG_CMD_BOOTM) += bootm.o
 ifeq ($(CONFIG_CPU_SH2),y)