Contribution description

This PR make the remaining RAM of AVR MCUs available as heap for malloc/free functions of avr-libc.

__malloc_heap_end is set to 0 by default. Therefore, malloc checks __brkval against the SP instead of __malloc_heap_end.

	cp = __malloc_heap_end;
	if (cp == 0)
		cp = STACK_POINTER() - __malloc_margin;
	if (cp <= __brkval)

In thread mode, however, the SP is is always in .data section below __bss_end and thus below __malloc_heap_start. Since __brkval points at least to __malloc_heap_start, the check always fails.

This PR sets __malloc_heap_end to the current SP before switching to thread mode. Thus, malloc checks __brkval against __malloc_heap_end.

Testing procedure

Use tests/malloc to check malloc/free functions of avr-libc, for example:

make -C tests/malloc BOARD=arduino-mega2560 PORT=/dev/ttyUSB0 flash test