build: improve tools/create-exports-NetworkManager.sh script

- include symbols from the "B" section.
- improve the script, to use libNetworkManager.a instead
  of the NetworkManager binary. The former is before stripping
  symbols.

tools/create-exports-NetworkManager.sh

@@ -29,8 +29,8 @@ call_nm() {
 }
 
 get_symbols_nm () {
-    call_nm ./src/NetworkManager |
-        sed -n 's/^[tTDR] //p' |
+    call_nm ./src/.libs/libNetworkManager.a |
+        sed -n 's/^[tTDRB] //p' |
         _sort
 }
 
@@ -56,7 +56,6 @@ pretty() {
 
 do_build() {
     do_update
-    touch src/main.c
     make
 }
 
@@ -70,9 +69,6 @@ do_update() {
 }
 
 do_generate() {
-    cd "$(realpath $(dirname "$0"))/.."
-    test -f ./src/NetworkManager
-
     cat <<EOF
 # this file is generated by $0
 {
@@ -87,6 +83,7 @@ EOF
 }
 
 
+cd "$(realpath $(dirname "$0"))/.."
 case "$1" in
     rebuild)
         do_rebuild