gpio: fix issue with gpio24, simplify blinky led pin

[mattthebaker]

Signed-off-by: Matt Baker baker.matt.j@gmail.com

[mattthebaker]

May require more testing, to see if it causes issues with other pins or peripherals. GP24/Green on m0sense now works out of the box.

[9names]

Note to self: check the readme where i documented the picocom flags needed to talk serial to xt-zb1 with it's inverted dtr and rts lines.