ESP32: Enable the allocation of Userspace heap exclusively in SPI RAM under Flat mode

[gustavonihei]

Summary

This PR intends to enable the separation of Kernel and Userspace heaps in ESP32 operating under Flat mode, by relying on the MM_KERNEL_HEAP extension brought by #8140.

This new feature enables the allocation of the entire Userspace heap into SPI RAM and reserving the Internal RAM exclusively for the Kernel heap, which may benefit some timing-constrained Kernel components (e.g. Wi-Fi driver).

Impact

Breaking change: ESP32_SPIRAM no longer auto selects XTENSA_IMEM_USE_SEPARATE_HEAP.
XTENSA_IMEM_USE_SEPARATE_HEAP is now selected by ESP32_IMM_HEAP, which in turn is not enabled by default.

Drivers that previously relied on XTENSA_IMEM_USE_SEPARATE_HEAP now bring in ESP32_IMM_HEAP if ESP32_SPIRAM is also selected, with the notable exception from ESP32_WIFI.

ESP32_WIFI previously used XTENSA_IMEM_USE_SEPARATE_HEAP for allocating dynamic data into Internal RAM via a specific API. With this PR, there is now an alternative approach using MM_KERNEL_HEAP. So it is up to the user to either select ESP32_IMM_HEAP or MM_KERNEL_HEAP.
In order to keep the current behavior, custom board users shall select ESP32_IMM_HEAP on Kconfig.

Testing

Successful execution of ostest in esp32-devkitc:psram_usrheap defconfig.
Successful connection to Wi-Fi network using a custom version of esp32-devkitc:wapi.