boston: Pad binary in .mcs to a multiple of 16 bytes

boston: Pad binary in .mcs to a multiple of 16 bytes

When flashing U-Boot on a Boston board using Xilinx Vivado tools, the
final 0x00 byte which ends the .relocs section seems to be skipped &
left in flash as 0xff unless the data contained in the .mcs is padded
out to a 16 byte boundary. Without our final zero byte relocation will
fail with an error about a spurious reloc:

Avoid this problem by padding out the data in the .mcs file to a 16 byte
boundary using srec_cat's -range-pad functionality.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>