buildroot

boost: add option for the stacktrace library

stacktrace requires dynamic library support, which was causing the
following build errors:

http://autobuild.buildroot.net/results/692ffad93a7bd867ecc7ccbfc8c6280735d29435/
http://autobuild.buildroot.net/results/6058ece804889abaaab0a29258e1de2904162d26/
http://autobuild.buildroot.net/results/12df9b345a90a4e011b8bb4cb1d1ef1c2c7040c0/
http://autobuild.buildroot.net/results/7473c433e93b3e785e44d9868fec517437f59847/

Adding an option for it allows to have it disabled by default, and
make sure it only gets enabled when shared library support is
available.

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