[FIX] Fix AVNET S6 MN demo linker scripts

Correctly assign ".local_memory" section to BRAM instance.

Change-Id: I5a24f1e255b993985e3861d8a4a2239a8cd5ff0c
[TASK] Increase local memory size for Xilinx MN designs

The local memory shall be used for performance optimization defined by
targetsection.h.
- LX150T design has 32 kB local memory available
- LX16 design has 16 kB local memory available

Change-Id: I4ab5bdedcf55749274dc63777c4144865299c634