Try to configure automatically GBDIR

GBDIR must be set in order to include the Greybus headers.
Since Greybus has been merged to mainline kernel,
we could just check if the current kernel >= 4.9,
and use it's sources to build gbridge.
If the kernel sources are not available,
then GBDIR could be set manually.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>