[FEATURE] Implement Kernel build for RPi 4B SMP

### Is your feature request related to a problem? Please describe.

The RPi 4B support currently uses a bundled image where there is only one memory region shared by the user and the kernel.

### Describe the solution you'd like

NuttX supports userspace/kernel space separation with kernel mode builds. I would like a kernel mode build to be supported for the Pi 4B with a correct linker script, configuration `defconfig` preset and some loaded tests to verify correct operation.

### Describe alternatives you've considered

_No response_

### Verification

- [x] I have verified before submitting the report.