boston: Add u-boot.mcs make target

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: Paul Burton <paul.burton@mips.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: u-boot@lists.denx.de