M2351: Re-build default secure image/gateway library to favor Pelion application

In this new memory partition, secure program is most simplified and non-secure program can make most use of memory for its large application like Pelion:

-   Flash (512KiB in total): 64KiB for secure and 448KiB for nonsecure.
-   SRAM (96KiB in total): 8KiB for secure and 88KiB for nonsecure.

Besides, to make secure program fit into 8KiB:

-   Decrease boot stack size to 0x600 bytes
-   Remove serial support

Author: ccli8