Add ability to select index of HW_TARGET for flashing on PCs with multiple JTAGs (default index 0)
This lets us flash to multiple FPGA permanent memories simultaneously without unplugging/replugging the JTAGs (for test beam mostly). Default behavior will not change.
Specify index by adding a second argument, i.e. ./flash.sh file.bit 1
to flash file.bit
to FPGA 1.