mm: Enable a dedicated kernel heap on BUILD_FLAT via MM_KERNEL_HEAP

[gustavonihei]

Summary

This PR intends to enable a dedicated kernel heap on NuttX Flat mode by extending MM_KERNEL_HEAP feature.

By separating the kernel and userspace heaps, the user is granted more control over the heaps placement within the memory hierarchy, which is specially useful for microcontrollers that provide External RAM.

Besides segregating the kernel and userspace allocations, this feature does not prevent the userspace from accessing the kernel heap under BUILD_FLAT.

MM_KERNEL_HEAP support under BUILD_PROTECTED and BUILD_KERNEL should be unaffected.

Impact

NuttX memory management subsystem is now extended with a new feature, enabled only on specific combination of Kconfig options.

Testing

Successful execution of ostest in esp32-devkitc:psram_usrheap defconfig, to be submitted later.

[gustavonihei]

CI failure is unrelated to this PR. Prior to rebasing it passed successfully, should we proceed with the merge or wait until #8155 is merged?

[xiaoxiang781216]

let's ignore the unrelated ci broken.