ZCU104: add I2C

Tested with:
./pano_logic_g2.py --uart-name=jtag_uart --build --load
./litex_jtag_uart.py --config=openocd_xc6_ft232.cfg
lxterm /dev/pts/X

Take Ethernet PHY out of reset so default clock is 125 Mhz

Tested with:
./forest_kitten_33.py --uart-name=jtag_uart --build --load
litex/tools/litex_jtag_uart.py
lxterm /dev/pts/X

platforms/nexys4ddr: add option to build with spi-mode sdcard support

platforms/ecp5_evn: rename spiflash1x to spiflash, rewrite hardware/configuration description and remove make_spiflash.

The Platform file should not contain import code related to cores, this has to be done in the target.

ECP5-EVN SpiFlash parameters

[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