buildroot

brotli: new package

Add the brotli package to Buildroot. This is needed by woff2, which in
turn is needed by webkitgtk from version 2.20.0 onwards. WebKitGTK+ used
to bundle a copy of the library, but it stopped doing so now that the
upstream has started making releases.

[Peter: fix license hash]
Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>