Apply DRY concept to gentoo recipe (and a base for DRY for other recipe?...

features :
- symlink to not architecture related content (.gentoo)
- symlink for common configuration (.common)
- possibility to choose sync country
- possibility to choose mirror select
- add the possibilty to choose to install ruby from source or portage
(with a bool)
- kernel_version is always the last
- add no-spinner to emerge
- a proper reboot script

I'm sorry for the "One commit" it might be difficult to read...

I'm waiting your comments @davidmmiller and @vaskas ;)