buildroot

pkg-waf: add a common variable for each step

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

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