Required to simplify simple.py target and use trellis/icestorm as default toolchain.

Also use new loose lookup_request to simplify constraints.

We initially wanted to provide different level of support for the platforms/targets, mainly
to avoid too much maintenance and let each contributor update its contributed platforms and
targets, but it's easier to update all platforms/targets all-together when LiteX evolves or
changes (and that's what has been done on litex-boards since the creation of the repository).
So let just simplify things and avoid this differentiation.

- use 1e9/freq for default_clk_period
- add default serial on tinyfpga_bx
- use S6PLL on minispartan6
- add SPIFlash pins on versa_ecp5