luci-mod-system: flash.js: add mtdblock name to filename

luci-mod-system: flash.js: add mtdblock name to filename

When you download a partition, the name is hostname+mtdX.bin
and that can be confusing if you go back and try to remember
what mtdX was if you have 10+ partitions.

So let us add the partition name to the filename like:
hostname.mtdX.boot.bin
hostname.mtdX.firmware.bin

Note: If there is a better, easier and simpler way to do the
same thing, please let me know.

Signed-off-by: Kristian Skramstad <kristian+github@83.no>