boost: context needs thread without C++11 mutex

Extract from output/build/boost-1.67.0/libs/context/build/Jamfile.v2:

explicit cxx11_hdr_mutex_check ;
local cxx11_mutex = [ check-target-builds
      cxx11_hdr_mutex_check "C++11 mutex"
    :
    : <library>/boost/thread//boost_thread
  ] ;

So select boost_thread if gcc <= 4.7

Fixes:
 - http://autobuild.buildroot.net/results/fb046c04fe18bec973d120e4ab33971f32ba5769



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>