libgpiod: new package

libgpiod: new package

Add a package containing a C library and a set of command-line tools
for controlling GPIOs from user space using the new character device
interface on linux.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
[Thomas:
 - add comment about autoreconf=yes (suggested by Romain Naour)
 - add more conventional syntax for the --{enable,disable}-tools usage
   (suggested by Romain Naour)
 - add patch to fix musl build.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>