[FREELDR] allow to boot with < 49MB of RAM

Assign a VM with limited amount of RAM. I tested with VBox 4.3.12 here (with disabled USB controllers).
if you spend 49MB it still passes, and all RosBE-Win2.1.6 gcc dbg livecds releases/0.4.7, releases/0.4.8, releases/0.4.9, releases/0.4.10, releases/0.4.11, releases/0.4.12, releases/0.4.13, releases/0.4.14 will pass the freeldr point and some of them even do reach the desktop with that (e.g. 0.4.14).

contrary if you spend only 48MB to the VM, the freeldr will immediately end the boot for all of them by immediately failing with

(../boot/freeldr/freeldr/lib/mm/mm.c:63) err: Memory allocation failed in MmAllocateMemory(). Not enough free memory to allocate 33554432 bytes.

A guy on youtube https://www.youtube.com/watch?v=eXQTT8XXiNU found out, that we can boot with much less RAM by changing the file mm.h the line

#define TEMP_HEAP_SIZE (32 * 1024 * 1024)

to smaller values like (in his extreme example)

#define TEMP_HEAP_SIZE (1 * 1024 * 1024)

We should credit "John M. from youtube" for that, and experiment a bit on our own to check whether there is some side-effect-less value to use here. Or at least get some inspiration from that.