Here are some exercises about lesson 08, where we add GPIO and printf.
Some are very simple, like exercise 5, some are not, as usual.
Exercise 6 is not related to this lesson, but it's interesting
(and a real problem with real code).

1- As requested in the video, please suggest changes to the better
in the GPIO API I committed. There are suboptimal details (try
to use it yourself, you will note).

2- Assuming the new API is incompatible, please suggest a way to
migrate from the current API to the new one. Imagine you have several
users of your current API, for example by replicating this include/ and lib/
in other packages (that want to track this "uspstream"), or that you
have dozens of applications in this very package (the two situations
are different, and the best approach differs).

3- Explain why the three leds in the TDC board are connected to ping
0_1, 1_28 and 1_31.

4- Suggest a way to add "external" GPIO pins (e.g. an I2C extender,
but that's irrelevant. What I want is an API to plug new GPIO pins
that are then accessible with the standard API (esp. gpio_set() and
gpio_get()).

5- Now that we have pp_printf, we really want to select among the
several implementations, instead of just using vsprintf-full.o. Please
fix the code base to do that.

6- With current code, we can use time_after() with jiffies, but we
could still do the buggy comparison "j < jiffies".  Please suggest a way to
force a build failure whenever the user touches jiffies as an integer,
to force consistent use of accessor functions (time_before(), but also
a new jiffies_add() that must be provided).

7- Why does tests-tdc/hello include GPIO code, even if it doesn't reference
GPIO and we use -gc-sections?