Vanilla FreeRTOS expects applications to use one of the heap implementations
provided by FreeRTOS (i.e., heap_x.c), where functions such as pvPortMalloc()
and vPortFree() are defined in the heap implementation.
However, ESP-IDF already provides its own heap implementation
(i.e., esp_heap_caps.h). Thus, the pvPortMallc()/vPortFree() functions were
previously overriden by macro to call esp_heap functions directly.
This commit refactors the FreeRTOS port's heap as such:
- Added a heap_idf.c that implements all of the heap related functions required
by FreeRTOS source
- All dynamic memory allocated by FreeRTOS is from internal memory. Thus, the
FreeRTOS heap is the internal memory subset of the ESP-IDF heap.
- Removed some old macros to reduce diff from upstream source code.
