Exercice 4.3 - Configuration without Standard Peripheral Library

File re-organisation :
For each exercice with no compatibility with the previous one (for
example 4.2 -> 4.3), a new file « exercice_[X.X].c », where [X.X] is
the number of exercice, is created.

Exercice 4.3:
Without the Standard Peripheral Library, toggle the green LED when the
user button is pushed.