userboot: allow building on !x86

[kevans]

Summary

We can still get plenty of use out of a userboot that doesn't know
anything about how to load or boot a kernel; notably, the test harness
in tools/boot can still be used to test lua changes.

Hack out the necessary bits to simply build on other platforms, and add
a small warning with ample time to view the warning on other platforms.
We still won't build userboot by default on these platforms, since the
build product isn't useful for most people.

[imp]

I'm cool with this...
It points out a bigger problem that we bogusly let userboot into the tree when it just did a cut and paste of the boot<->kernel handoff code w/o forcing a refactor.
But that's beyond the scope of this...

stand/userboot/userboot/main.c

210

that delay seems too long for testing...

[kevans]

In D41529#946424, @imp wrote:

I'm cool with this...
It points out a bigger problem that we bogusly let userboot into the tree when it just did a cut and paste of the boot<->kernel handoff code w/o forcing a refactor.
But that's beyond the scope of this...

thanks! yeah, I wouldn't mind seeing some improvement here, though I figured that could eventually be needed/done anyways as part of a larger project if a later iteration of arm64 bhyve wants to try and support it for whatever reason.

stand/userboot/userboot/main.c

210

I played around with it a little bit; I think dropping it by a second to .5 seconds would probably be sufficient to get the point across without adding too much latency to the feedback process.

[imp]

stand/userboot/userboot/main.c

210

Oh. I miscounted zeros. 1.5s is fine. I thought 15s.