Add open-source toolchain support for GateMate

[meiniKi]

Adds nextpnr-himbaechel support for GateMate to edalize. Currently, edalize only supports the proprietary p_r tool for place&route.

Support for GateMate by nextpnr is added as a separate .py file instead of adding options to gatemate.py for the following reason:

• No change in the existing API. Even if a pnr option with a default value is added as in icestorm, the option p_r_options is no longer descriptive to carry the device information for nextpnr.
• Consistent naming convention: peppercorn for GateMate; as trellis for ECP5, icestorm for iCE40, and apicula for Gowin.
• Keeping required yosys flags separate: -luttree, -nomx8

Example fusesoc target:

  <name>:
    default_tool: peppercorn
    description: <desc>
    filesets: <filesets>
    parameters: <param>
    tools:
      peppercorn:
        device: CCGM1A1
        gmpack_options: [--crcmode, ignore]
    toplevel: <top>

[meiniKi]

Overlooked your #438 (comment). The same applies to this.

[olofk]

Yes, I was just about to comment on that :) Other than that I'm excited to see this addition. Thanks for working on it.