DATA_DIR override with environment variable

[mmicko]

Would it be possible to add ability to override DATA_DIR location with environment variable ( OPENFPGA_LOADER_DIR for example) ? With oss-cad-suite can place their files anywhere, so using environment variable or checking it relative to binary position are only options.

https://github.com/trabucayre/openFPGALoader/blob/master/src/xilinx.cpp#L472

[trabucayre]

Is this solution looks good to you?
With this commit if you set OPENFPGALOADER_SOJ_DIR to priovides an alternate spiOverJtag bitstreams directory, this directory will be used, instead of DATA_DIR to open the bridge.

[mmicko]

Thanks @trabucayre this looks just right. Will work on change required on oss-cad-suite tomorrow.

[mmicko]

Thanks, all working now as expected