score: Fix CPU context usage on SMP

We must not alter the is executing indicator in
_CPU_Context_Initialize() since this would cause an invalid state during
a self restart.

The is executing indicator must be valid at creation time since
otherwise _Thread_Kill_zombies() uses an undefined value for not started
threads.  This could result in a system life lock.
bsp/gen83xx: Disable interrupt nesting for br_uid

This is necessary for the USB support.