rtt: Disable spurious warning in optimized build
rtt: Disable spurious warning in optimized build

With only CONSOLE_RTT enabled (other RTT features disabled) there is
only single up/down buffer thus buffer index !=0 is invalid. When
building RTT target code with optimizations enabled GCC emits an "array
subscript is above array bounds" error (-Werror=array-bounds) at line
1422 below because this code is accessed only if BufferIndex is larger
than 0 (thus out of array bounds). However, this will not happen due to
check in line 1419 as MaxNumUpBuffers is set to array size.

1419   if (BufferIndex < (unsigned)_SEGGER_RTT.MaxNumUpBuffers) {
1420     SEGGER_RTT_LOCK();
1421     if (BufferIndex > 0u) {
1422       _SEGGER_RTT.aUp[BufferIndex].sName        = sName;

Since there is external code which we don't want to modify and also
there is no error in code, let's just disable this warning for RTT
package.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core