extmod/btstack/btstack.mk: Add -Wimplicit-fallthrough=0.
extmod/btstack/btstack.mk: Add -Wimplicit-fallthrough=0.

This is needed since -Wextra was added to the build in
bef412789ea93c521bd9c2dddc22b9b3484da574

Signed-off-by: Jim Mussared <e84f5c941266186d0c97dcc873413469b954847e@gmail.com>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython