heap usage report

[mkiiskila]

We should have a console command/newtmgr operation to query heap use. I'd want minimally to see a) how much space was in the heap initially and b) how much was the max allocated amount.

Code should live next to _sbrk() implementation for the MCU, and the API could be optional to implement (presence of the implementation would be advertised with syscfg).

Optionally add a interface to baselibc which reports amount of memory in freelist. Not all use targets use baselibc (native targets, specifically), so including that data within the heap report could depend on whether baselibc is used or not.