Switch to using openFPGALoader as the default tool to upload bitstreams to xc7 devices

[umarcor]

In the documentation of arch-defs, guidelines were updated to recommend using openFPGALoader:

• https://f4pga.readthedocs.io/projects/arch-defs/en/latest/getting-started.html#openfpgaloader
• docs: Introduce OpenFPGALoader programming tool f4pga/f4pga-arch-defs#2240

However, make download in this repo is using openocd:

• https://f4pga-examples.readthedocs.io/en/latest/building-examples.html#counter-test
• https://github.com/chipsalliance/f4pga-examples/blob/main/common/common.mk#L78-L88

Compared to openFPGALoader, openocd does need a configuration file to be generated/provided, so it's cumbersome to use. There are use cases which openocd supports and openFPGALoader does not, but those are not related to uploading the bitstreams. Hence, we should update the makefile and environment in this repo to use openFPGALoader.

Ref: hdl/containers#60