The current logic failed silently on some systems, maybe because of a
different default shell? This resulted in builds that completed
successfully but generated invalid UF2 files, which were refused by
the uC bootloader.
Now the check is properly enforced and picotool is either found in
the $PATH or built from the pico-sdk source.
Signed-off-by: Niccolò Maggioni <nicco.maggioni+nuttx@gmail.com>
