This adds optional usage of periph_flashpage_raw for the
riotboot/flashwrite module. This removes the need to buffer a full
flashpage page, instead it must buffer two times the
FLASHPAGE_RAW_BLOCKSIZE. One is used to buffer the current write block,
the other buffers the first chunk (offset zero, page zero). This first
chunk is written when finalizing the flash operation.
Care must be taken that, when using the skiplength, the number of bytes
skipped is always a multiple of the FLASHPAGE_RAW_BLOCKSIZE.
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.
