add pru-programmer and preliminary gcc-support

Changes:

• merge programmer-branch (thanks to @geissdoerfer)
  • currently alpha, as shepherd is not able to switch between pru-firmwares
• add gcc-support (thanks to previous work of @fedy0)
  • make switches to gcc as soon as PRU_GCC is defined
  • currently beta! as pru0-firmware does not fit into IMEM
• shepherd_herd
  • fix and modernize shepherd-herd
  • add testbench
  • refactor code
  • extend functionality: distribute files, start / stop / check measurements
• change interface for CLI
  • enable_io (was set_target_io_lvl_conv or similar)
  • io_target and pwr_target (was io_target_a and b, or io_sel_target_a, same for pwr
• sheep detects missing kernel-module and missing sudo and fails early
• added doc for herd
• added doc for shepherd-v2 improvements
• ansible
  • refactor role "dev-host" into sheep
  • add installer for GCC-Toolchain
  • remove duplicate code (dev_rebuild can now be done by sheep-role)
  • modernize some loops
  • fix reboot-hang
• github action
  • now builds pru-firmwares (including programmer)
  • now also upload shepherd_herd to pypi

tested on:

• shepherd-sheep testbench
• shepherd-herd testbench
• ansible install on 11 nodes

trouble:

• PRUs seem more unstable in the big herd of 11 (sync is oscillating, state is rarely stuck in reset)
• switching to programmer-firmware is not implemented yet

compile-sizes (CGT vs GCC)

• pru0-fw 85020 byte, 91660 byte (IMEM overflow)
• pru1-fw 63820 byte, 42644 byte
• prog-swd 80108 byte, 68176 byte
• prog-sbw 83620 byte, 72900 byte