manual: explicitize manual license

People reading the Buildroot manual online cannot immediately know what its
license is (unlike reading it from within the source tree). To avoid any
discussion, explicitize this in the manual.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>