netsurf-buildsystem: new package

This is the NetSurf project shared build system, used by various NetSurf
sub-projects like the libsvgtiny project.

[Thomas: use cp -dpfr instead of just cp -r, to match what we do in
other Buildroot packages.]

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>