cerbero: Rework bootstrap options a bit

We have three bootstrap stages:

1. Platform-specific steps (on Linux, use the package manager, on
   Windows install MinGW, for Android install the NDK, etc)

2. Install the Rust toolchain (if the variant is enabled)

3. Build and install 'build-tools' recipes

"(no-)build-tools" and "(no-)system" as toggles are easier to
understand than "only build tools" (disable (1) and (2)), etc.