Add support for LiteX and friend repositories

[mithro]

• https://github.com/enjoy-digital/

The "big three" would be;

• [ ] litex
• [ ] liteeth
• [ ] litedram

The smaller ones would be;

• [ ] litepcie
• [ ] litescope
• [ ] litevideo
• [ ] litesata
• [ ] liteusb

[mithro]

Example migen receipe at https://github.com/nathanrossi/meta-hdl/commit/b8b45708ccbfc95bae4716592894b5f58a9b6042#diff-f98e6fb1f28d2f22cf691c93008fef91

[nathanrossi]

Recipes for litex and lite* cores (excluding liteusb which is deprecated) are now available in master.