Add to minitest/litex all the LiteX related minitests.

• min - Minimal configuration - just a CPU + uart targeting the Basys + Arty boards.

Basys

• CPU configured in "lite" mode? -- IE No DSPs or anything.
• CPU configured with divider + multiplier + etc -- with DSPs enabled.

Arty

• CPU configured in "lite" mode? -- IE No DSPs or anything.
• CPU configured with divider + multiplier + etc -- with DSPs enabled.

• min_ddr - Minimal configuration with DDR - CPU + DDR + uart targeting the Basys + Arty boards.

Basys

• CPU configured in "lite" mode? -- IE No DSPs or anything.
• CPU configured with divider + multiplier + etc -- with DSPs enabled.

Arty

• CPU configured in "lite" mode? -- IE No DSPs or anything.
• CPU configured with divider + multiplier + etc -- with DSPs enabled.

• base - SoC config with DDR and Ethernet targeting the Arty.

• CPU configured in Linux mode but with no DSPs.
• CPU configured in Linux mode but with DSPs.

Add a README which describes how to generate these outputs or even add a script which generates them.
We should also capture the git revisions of everything used to generate this output.

In the future we can expand to adding;

• SoC config with DDR, PCIe + HDMI for NeTV2.
• SoC config with DDR, DisplayPort + HDMI + GigE for Nexys Video (200T part)