Take into account stack buffers and printf for allocating the default
stack size.
This solves stack size issues with `wsn430-v1_3b` and `z1`.
It now have a main stack usage of 514 bytes out of 768 on `wsn430-v1_3b`.
Added arduino-nano to BOARD_INSUFFICIENT_MEMORY/BOARD_BLACKLIST following suit
of how arduino-uno is marked, as arduino-nano is mostly an Uno in a different
form factor.
When running the test on `arduino-mega2560` printing the float failed
and was printed as ` ?`.
Calculated ? bits of entropy from 10000 samples.
Replace using `printf` floating point printing by using `fmt/print_float`.
Now the test succeeds on `arduino-mega2560`.
As `print_float` does not buffer and is used with `printf` the output
should be flushed before calling it if `fflush` is available.
