Add a "flash" target to ECP5 EVN makefiles

[shreekumar3d]

blinky example's Makefile has the "make prog" target. This doesn't program the flash. When power is removed & reconnected, the bitstream that was flashed earlier gets loaded. This is confusing - for beginners at-least.

The openFPGALoader tool works with the EVN. Adding this as an optional flashing step in the makefile would help those starting off. e.g., the "flash" target would look like:

flash: ${PROJ}.bit
    openFPGALoader -f $<

[shuckc]

You can also accomplish the same using Greg Davill's ECPprog tool https://github.com/gregdavill/ecpprog

Lattice Deployment tool can also generate an SVF file to do background programming per
Programming External SPI Flash through JTAG for ECP5/ECP5-5G Technical Note FPGA-TN-02050
https://latticesemi.com/view_document?document_id=52228

Many years ago I added PLD support to OpenOCD for programming & reconfiguring ECP3 without needing to generate SVF, maybe I should get the patch up to date and add a flashing mode https://github.com/shuckc/urjtag/pull/4/files . The benefit of doing this 'within' OpenOCD is you get support for all the flash models for free.