Teach nanobsd about armv7, add -m to qemu invocations

[kevans]

Summary

qemu defaults to 128 MiB of RAM, which has been found to not necessarily be enough for booting the system, at least on amd64 and armv7

Add a sensible -m 512 to the examples so that they'll work out of the box in the general case.

[imp]

I have a few of these that conflict a little, but go ahead and commit these and I'll sort that all out.