Sort include alphabetically and make them in order:
Main header
system headers
library headers
local headers
While reordering, change a include of endian.h to our sparse-endian.h
which is more reliant to toolchain changes.
The initial idea was to export all pins to be used at once, so we
created _export_pins() to take all of them and a wrapper method,
_export_pin() to export a single one. However we never export more than
one pin at once.
Make sure pinMode() method exports the pin before using it. Otherwise
the export method would need to be called, differently from other GPIO
implementations.
Since now export_pin and export_pins are called only internally, reduce
their visibility to protected.
This commit adds the class Linux::GPIO_Sysfs. This class provides a generic
implementation of AP_HAL::GPIO on Linux by using GPIO Sysfs Interface
(https://www.kernel.org/doc/Documentation/gpio/sysfs.txt).
The channel() interface should be preferred in places that need to be
fast. Since it maintains the file descriptor open this is much faster
than opening and closing it.
Lucas De Marchi
José Roberto de Souza
Gustavo Jose de Sousa