docs/manual: slightly clarify patch licensing

Adding a patch is a modification to Buildroot. Thus this sentence might be
interpreted as "Patches are released under the Buildroot license".

Being a legal matter, the last thing we want is to be
ambiguous. Reword the sentence to avoid any misunderstandings.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>