shared: Fix compiler warnings in buxtonsimple for GCC 4.8.1

Due to the use of proxied macro prints, the compiler is under the impression
that the variables are unused before printing in buxton_debug. As opposed to
clearly marking them __attribute__ ((unused)) and causing confusion, we
explicitly tag them as "_bxt_used_", which in reality is defined as
__attribute__ ((unused)) - however it keeps the usage within the code more
logical.

Signed-off-by: Ikey Doherty <michael.i.doherty@intel.com>