Implement MK_NVME now that the expression for where NVMe is
complicated. Default it to "yes" for x86 and powerpc64 and
no everywhere else. Use it in camcontrol to define WITH_NVME
for those platforms where we support nvme.

This should fix the newly introduced nvme files to camcontrol
which were building everywhere.

Pointy Hat To: imp
Sponsored by: Netflix