Standardize the `cpu_variant` strings.

Standardize the `cpu_variant` strings.

Current valid `cpu_variant` values;
 * minimal  (alias: min)
 * lite     (alias: light, zephyr, nuttx)
 * standard (alias: std) - Default
 * full     (alias: everything)
 * linux

Fully documented in the [docs/Soft-CPU.md](docs/Soft-CPU.md) file
mirrored from the
[LiteX-BuildEnv Wiki](https://github.com/timvideos/litex-buildenv/wiki).

Also support "extensions" which are added to the `cpu_variant` with a
`+`. Currently only the `debug` extension is supported. In future hope
to add `mmu` and `hmul` extensions.