* Add Cortex-M4 toolchains with floating point hardware instructions.
* Update stm32f429i-disc1 to use hardware fpu toolchain by default.
* Add code to enable FPU in pw_dumb_io.
Verified tests that use floating point operations continue to pass.
Change-Id: I76bc7eeaf457eca1abacbc60992648d63dc85bf2
Add/update docs for the pw_dumb_io facade and backends. Configures GN
build files such that all the backend docs are built even if the backend
isn't in use.
Change-Id: I91fca1cb2d3f5d4b57ae1f6991a89270a35be1c6