usr.sbin/ntp: don't emit versions w/ make -s

<sys.mk> defines ECHO=echo when not using make -s, and ECHO=true when using
make -s.

export ECHO for ntp products and use it in the mkver script to echo the
version. This suppresses the output as appropriate. ECHO is given a default
value to make sure things still work as expected for anyone that isn't
redefining ECHO.

Reviewed by: cy
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22101