build: allow configuring path to binutils's nm tool during build

`nm` is used by "tools/create-exports-NetworkManager.sh" script.
Alloc configuring an explicit path during configure.

  BINUTILS_NM=/usr/bin/nm ./configure

tools/create-exports-NetworkManager.sh

@@ -29,7 +29,7 @@ _sort() {
 }
 
 call_nm() {
-    nm "$1" |
+    "${BINUTILS_NM:-${NM:-nm}}" "$1" |
         sed -n 's/.* \([^ ]\) \([^ ]*\)$/\1 \2/p'
 }