buildroot

pkg-waf: add additional variables for build step

Some waf packages may want to pass additional variables to waf script in
build step. Add the possibility to do so by defining <pkg>_BUILD_OPTS.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>