Fix a small bug in the -sysdir handling.

The issue was that auto-configuration would set the value of -system
to point to the full path of the system image (e.g. <sysdir>/system.img).

Later, the emulator would try to load the <sysdir>/<sysdir>/system.img due
to this.

Change-Id: Id77ea9bdc55bb99c1bfbd2ade291ec41f31ec65d
diff --git a/android/main.c b/android/main.c
index cd3fd08..f20ea7b 100644
--- a/android/main.c
+++ b/android/main.c
@@ -549,7 +549,7 @@
             }
 
             /* If -system <name> is used, use it to find the initial image */
-            if (opts->sysdir != NULL) {
+            if (opts->sysdir != NULL && !path_exists(opts->system)) {
                 initImage = _getFullFilePath(opts->sysdir, opts->system);
             } else {
                 initImage = ASTRDUP(opts->system);