sh4: use single u-boot linker script for all boards

Three supported SH4/SH4A boards with the bootloader image stored on
SPI flash have own flavour of a linker script, in turn they are equal
among each other. The only difference is that the text from
lowlevel_init.o is placed right after start.o, which makes sense.

Note that .bss section is not marked as NOLOAD, because for about
10 years this is a default option of a GNU linker, either the
attribute is found or not the resulting image file is the same.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/board/renesas/sh7753evb/Makefile b/board/renesas/sh7753evb/Makefile
index f7c8e94..4293142 100644
--- a/board/renesas/sh7753evb/Makefile
+++ b/board/renesas/sh7753evb/Makefile
@@ -4,4 +4,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 
 obj-y	:= sh7753evb.o spi-boot.o
-obj-y	+= lowlevel_init.o
+extra-y	+= lowlevel_init.o