buildroot

docs/manual: misc improvements to Vagrant usage documentation

* Changing back wget to curl cause wget it's not available
  on mac os x.
* Braking up too long windows one liner.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>