boost: enable boost-atomic

It was disabled in february 2013 by commit
e5434583
because did not build correctly with ucLibc at the time.

It now builds correctly with both uClibc v0.9.33 and uClibc-ng.

Signed-off-by: Noé Rubinstein <nrubinstein@aldebaran.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>