[WIP] Add Alveo U250 platform and target

- add copyrights.
- add link to documentation.
- reindent code.
- rename some IOs (for consistency with the others platforms).
- add pullup on clk20_vcxo_en.
- set clk20_vcxo as default clock.
- remove ununnecessary eth constraints. (we are using IO primitives).
- move PMODS to connectors (for consistency with others platforms).

Marblemini [https://github.com/berkeleylab/marble-mini/]

Default to Chaser mode and similar user interface than GPIOOut.

targets/orangecrab.py: propagate command arguments

community/platforms/marblemini.py: Added marblemini from https://github.com/berkeleylab/marble-mini/

The parsed args are stripped off by soc_core_argdict() (called from
soc_sdram_argdict() so we have to pass them explicitly (or pass the
original "args", but this mimics the rest of the code in the repo).

This fixes #72

Also use new loose lookup_request to simplify constraints.

platforms: make sure all Xilinx/Altera platforms have a create_programmer method, use OpenOCD on Spartan6 and 7-Series.

As discussed recently on Discord.

Signed-off-by: David Shah <dave@ds0.me>