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

usr.sbin/ntp/Makefile.inc

@@ -16,4 +16,6 @@ CFLAGS+= -DOPENSSL -DUSE_OPENSSL_CRYPTO_RAND -DAUTOKEY
 
 WARNS?=	0
 
+.export ECHO
+
 .include "../Makefile.inc"

usr.sbin/ntp/scripts/mkver

@@ -4,6 +4,8 @@
 #
 PROG=${1-UNKNOWN}
 
+: ${ECHO:=echo}
+
 ConfStr="$PROG"
 
 ConfStr="$ConfStr 4.2.8p12"
@@ -32,7 +34,7 @@ echo $RUN > .version
 
 ConfStr="$ConfStr (${RUN})"
 
-echo "Version <${ConfStr}>";
+${ECHO} "Version <${ConfStr}>";
 
 rm -f version.c
 cat > version.c << -EoF-