Currently, if the user wants to allocate heap on external RAM
he needs to enable CONFIG_ESP_SPIRAM and set a threshold defined
with CONFIG_ESP_HEAP_MIN_EXTRAM_THRESHOLD.
This approach requires that we re-implement `k_malloc` and allocate
the memory on the proper region based on the block size.
By using the shared multi heap feature the proccess of allocating
memory from external memory becomes more fluent and simple.
The attribute SMH_REG_ATTR_EXTERNAL was added to reference the
external memory.
Signed-off-by: Lucas Tamborrino <lucas.tamborrino@espressif.com>
5dc545290c