carl9170 firmware: add more compiler flags voodoo

"Link Time Optimization (LTO) gives GCC the capability of
dumping its internal representation (GIMPLE) to disk, so
that all the different compilation units that make up a
single executable can be optimized as a single module.

This expands the scope of inter-procedural optimizations to
encompass the whole program (or, rather, everything that is
visible at link time)."

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>