RISC-V: Check that the DTB doesn't overlap with kernel

[arichardson]

Summary

This can happen with very large kernels (e.g. ones embedding a root
filesystem). The DTB written by OpenSBI/BBL is quite small so this is
unlikely to hit important data, but if it does this can result in very
confusing and hard-to-debug crashes. Add a KASSERT() and a verbose print
to catch this problem with debug kernels.

While this will not print any output by default if it fails (that would
depend on EARLY_PRINTF), at least the kernel now halts reliably instead
of randomly crashing.

[mhorne]

sys/riscv/riscv/machdep.c

783 ↗

(On Diff #72725)

This secondary indentation is unusual, but I think it's the right thing to do here.

[arichardson]

sys/riscv/riscv/machdep.c

783 ↗

(On Diff #72725)

This is caused by clang-format. I'm never quite sure how to correctly indent nested statements, so I just pick the clang-format output.