Add an implementation that waits for 's' to print 'START' and return.
If 'r' is given is prints 'READY' to allow querying for state.
The help and answered string have to be different to not match the other.
Using puts/getchar was smaller than using `stdio_read/stdio_write` on the
example I tested with `esp32`.
Add a trivial shell program that scans for all slaves on an I2C bus by iterating
all of the possible 127 I2C addresses and checking for the ACK of the device.
