west: runner: add support for ecpprog

The ecpprog command is an utility written by Greg Davill for flashing
FPGAs such as ECP5 or CrossLink-NX series. Devkits typically have an
FTDI interface chip to access the external flash. FPGA image is
typically at flash offset 0x00000000 flash offset, and the Zephyr
image offset can be set via CONFIG_FLASH_LOAD_OFFSET.

Signed-off-by: Josuah Demangeon <me@josuah.net>