Move app stack to beginning of RAM

[dehanj]

• Fail on stack overflow, due to security monitor
• Explicitly defines stack size
• Affects user defined CPU_MONITOR, which monitors execution in desired area
• Investigate if this breaks current API/app_size/app_addr etc.

[mchack-work]

It would be very nice to have stack overflow protection by hardware, for instance by moving the stack to the beginning of the memory.

The current Security Monitor can, however, only cover one continous area in app RAM. It is currently typically used by an app like this:

	// Use Execution Monitor on RAM after app
	*cpu_mon_first = *app_addr + *app_size;
	*cpu_mon_last = TK1_RAM_BASE + TK1_RAM_SIZE;
	*cpu_mon_ctrl = 1;

which means all RAM after the loaded app, typically .text and .data, is covered by the Security Monitor from execution. This means .bss, any heap that might be in use, and the stack are protected from execution.

If we just move the stack to the beginning of RAM, the .bss and heap will become unprotected from execution.