Moved secondary heap init code to flash.
  External -24 IRAM, +32 IROM
  IRAM     -76 IRAM, +64 IROM

General updates to umm_init call path and DEFINES to better align with
upstream. Name changes: UMM_INIT_HEAP with UMM_CHECK_INITIALIZED,
umm_init_stage_2 with _umm_init_heap, and umm_init_common with umm_init_heap.

Add file umm_cfgport.h to hold port-specific values. Stay focused
on heap initialization only move-related defines.

Improved comments.

Created a wrapper function for running pre-SDK code from flash.
Updated hwdt_app_entry to use it.

Update umm_init with option to run from ICACHE.
Added build define UMM_INIT_USE_ICACHE to move umm_init call path to flash.
When used frees up 160 bytes of IRAM at a cost of 208 bytes of IROM

Defaults to no change, umm_init call path will be in IRAM.

…to UMM_INIT_USE_IRAM.