boost: do not allow context module on sparc

The boost context module does not build on sparc, even if sparc is
advertised as a supported architecture by the boost documentation. So
let's disallow this module for the time being.

Fixes:

  http://autobuild.buildroot.org/results/206/2060e6e31c4d739947097faf6587e0a06681fee2/



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