build/export: correct the export path

Change-Id: Ia2e19f30d32154c3329f46c5d8362fafd6b7e274
Signed-off-by: chao.an <anchao@xiaomi.com>

tools/mkexport.sh

@@ -432,7 +432,7 @@ cd "${TOPDIR}" || \
   { echo "MK: 'cd ${TOPDIR}' failed"; exit 1; }
 
 if [ -e "${APPDIR}/Makefile" ]; then
-  "${MAKE}" -C "${TOPDIR}/${APPDIR}" EXPORTDIR="$(cd "${EXPORTSUBDIR}" ; pwd )" TOPDIR="${TOPDIR}" export || \
+  "${MAKE}" -C "${APPDIR}" EXPORTDIR="$(cd "${EXPORTSUBDIR}" ; pwd )" TOPDIR="${TOPDIR}" export || \
       { echo "MK: call make export for APPDIR not supported"; }
 fi