boston: Add u-boot.mcs make target

U-Boot is generally flashed to a MIPS Boston development board by means
of a .mcs file which Xilinx Vivado software can write to the flash
present on the board. As such we'd generally want to produce an mcs file
when building U-Boot to target the Boston board. Introduce a make target
for u-boot.mcs which generates it using the srec_cat tool available from
the SRecord project, and build it by default when srec_cat is present.
Signed-off-by: NPaul Burton <paul.burton@mips.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: u-boot@lists.denx.de

board/imgtec/boston/config.mk

+#
+# SPDX-License-Identifier:	GPL-2.0+
+#
+
+quiet_cmd_srec_cat = SRECCAT $@
+      cmd_srec_cat = srec_cat -output $@ -$2 $< -binary -offset $3
+
+u-boot.mcs: u-boot.bin
+	$(call cmd,srec_cat,intel,0x7c00000)
+
+# if srec_cat is present build u-boot.mcs by default
+has_srec_cat = $(call try-run,srec_cat -VERSion,y,n)
+ALL-$(has_srec_cat) += u-boot.mcs
+CLEAN_FILES += u-boot.mcs