libmodprobe: add support for a blacklist

If the blacklist is enabled, blacklisted modules are treated as though
they are not present.

Change-Id: Ie8712f24298e78f92d5028b1ca3a8a3e07a9190a
diff --git a/libmodprobe/libmodprobe_ext_test.cpp b/libmodprobe/libmodprobe_ext_test.cpp
index 204809f..7d817b1 100644
--- a/libmodprobe/libmodprobe_ext_test.cpp
+++ b/libmodprobe/libmodprobe_ext_test.cpp
@@ -67,6 +67,9 @@
 
 bool Modprobe::ModuleExists(const std::string& module_name) {
     auto deps = GetDependencies(module_name);
+    if (blacklist_enabled && module_blacklist_.count(module_name)) {
+        return false;
+    }
     if (deps.empty()) {
         // missing deps can happen in the case of an alias
         return false;