boost: ignore site config

It can lead to build failures if you've got boost installed in the host,
like this:
http://stackoverflow.com/questions/23013433/how-to-install-modular-boost



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>