targets: add simple design (vendor agnostic and usable on all platforms with UART pins).

Designing a SoC with Migen is easy, but we have to provide a very simple design that can
be used on all boards with only 1 clock and 2 UARTs pins defined. This will encourage the
newcomer to invest time in Migen/MiSoC and see its real potential.
update README with new Kintex-7 support