Split bootstrap --system-only / --build-tools-only into system-deps / extract-toolchain / build-tools
--system-only currently does 'install system deps' (for platform) and 'extract toolchain' (for target_platform). This is bad because:
- Installation of system deps is an infrequent task
- Installation of system deps requires root on Linux
- Installation of system deps needs to be done in the Dockerfile when building the image
- Extraction of toolchain is a frequent task
- Extraction of toolchain needs to be done in CI jobs, not when building the image
The new options must allow picking one, two, or all of these tasks.