buildgcc: use ${GDB_DIR} instead of gdb-${GDB_VERSION}

Bring gdb in sync with all other build targets.

Change-Id: I9c478947a00f044edf910a91d876bbf486a791cf
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10488
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>

util/crossgcc/buildgcc

@@ -424,7 +424,7 @@ build_GDB() {
 	LDFLAGS="-Wl,-rpath,\$\$ORIGIN/../lib/ -L$DESTDIR$TARGETDIR/lib \
 		 -lpthread $LIBDL -lutil" \
 	CC="$CC" CFLAGS="$HOSTCFLAGS -I$DESTDIR$TARGETDIR/include" \
-	../gdb-${GDB_VERSION}/configure --prefix=$TARGETDIR \
+	../${GDB_DIR}/configure --prefix=$TARGETDIR \
 		--target=${TARGETARCH} --disable-werror --disable-nls
 	$MAKE $JOBS || touch .failed
 	$MAKE install DESTDIR=$DESTDIR || touch .failed