Add SVNVERSION_CMD to bsd.own.mk, adding the capability to include
svnversion metadata to the runtime and kernel packages.

Instead of traversing src/sys, as is done by newvers.sh for uname(1),
a full tree walk is done to prevent userland and/or modifications
from not being reflected in a modified tree (M).

MFC after: 5 days
Sponsored by: The FreeBSD Foundation