Memory manager to safely use malloc() from other libraries

[ppescher]

The current options in the portable/MemMang directory cannot provide thread safety for those Arduino libraries that directly call malloc() family of functions (e.g. STM32SD, String).

I have ported some code here: RoboTech-srl@897009a

Before making a PR, I was wondering if configMEMMANG_HEAP_NB == -1 was a good choice. I thought the number of heap implementations might grow in future versions, so I went negative. What do you think @fpistm ?

I also think that the default _sbrk() provided by the core must be defined as a "weak" symbol for this to work. See fortebit/fortebit-openiot-stm32-core@169ced0

[fpistm]

Thanks @ppescher
I will checkout this tomorrow, I'm currently working on an issue around malloc #8.
As I pointed in the thread and during my investigation I've planned to give a try to this heap_useNewlib.c which is provided in the link I've post to @rayozzie.
Nice to have your proposal.