freebsd-update: Use BASEDIR when checking for src component

src could potentially be installed under the based dir
and not under the root or vice versa.

PR:		224048
Submitted by:	Gerald Aryeetey <aryeeteygerald_rogers.com>
Reviewed by:	delphij
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D18849

usr.sbin/freebsd-update/freebsd-update.sh

@@ -221,7 +221,7 @@ config_KeepModifiedMetadata () {
 config_Components () {
 	for C in $@; do
 		if [ "$C" = "src" ]; then
-			if [ -e /usr/src/COPYRIGHT ]; then
+			if [ -e "${BASEDIR}/usr/src/COPYRIGHT" ]; then
 				COMPONENTS="${COMPONENTS} ${C}"
 			else
 				echo "src component not installed, skipped"