FreeLoader: Obtain memory map via multiboot on Xbox

Currently FreeLoader synthesizes the Xbox memory map by itself in XboxMemGetMemoryMap.

The actual memory map can be obtained using multiboot 1 specification, since on Xbox we use Cromwell bootloader which passes multiboot record (including the memory map) to the FreeLoader.

This would be the proper way of getting the Xbox memory map, and also it will help to define additional special memory regions at the Cromwell side for such things as DMI/SMBIOS table and firmware callback interfaces.