[sw] Move build system to Meson

[moidx]

Replace software Makefiles with Meson build files. The work is broken down as follows:

• Add support for Meson. Starting with:
  • sw/boot_rom
  • sw/lib
  • sw/host
  • sw/tests
• Update documentation with new build instructions, e.g.:

  $ cd ${REPO_TOP} && ./meson_init.sh
  # Verilator
  $ cd ${REPO_TOP}/build-sim && ninja
  # FPGA
  $ cd ${REPO_TOP}/build-fpga && ninja

• Update documentation with new dependencies:

  $ pip3 install --user meson
  $ apt-get install ninja-build

• Update Docker container to support new dependencies
• Update otx to support new dependencies
• Update other build targets, e.g. sw/benchmark
• Update CI to trigger build-sim and build-fpga builds

[aytong]

@moidx, can you update checkboxes to show what's left? Thanks.

[mcy]

See #1032.