Teach nanobsd about armv7, add -m to qemu invocations

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.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D14234