config(8): Fix broken ABI

r336019 introduced ${SRCTOP}/sys to the include paths in order to pull in a
new sys/{c,}nv.h. This is wrong, because the build tree's ABI isn't
guaranteed to match what's running on the host system.

Fix instead by removing -I${SRCTOP}/sys and installing the libnv headers
with make -C lib/libnv includes... this may or may not get re-worked in
the future so that a userland lib isn't installing includes from sys/.

Reported by: bdrewery