Fix running binwalk from build dir

This patch fixes some cases in running binwalk:

 - from build dir: src/scripts/binwalk
 - from repo dir: src/build/binwalk
 - from custom dir, but without full path: ./binwalk
   (`dirname(dirname('./binwalk'))` becomes an empty string which is
   treated as "current working directory" while the expected output is
   "..")

src/scripts/binwalk

@@ -7,11 +7,19 @@ from threading import Thread
 # If installed to a custom prefix directory, binwalk may not be in
 # the default module search path(s). Try to resolve the prefix module
 # path and make it the first entry in sys.path.
-_module_path = os.path.join(os.path.dirname(os.path.dirname(__file__)),
-                            "lib",
+# Ensure that 'src/binwalk' becomes '.' instead of an empty string
+_parent_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+for _module_path in [
+    # from repo: src/scripts/ -> src/
+    _parent_dir,
+    # from build dir: build/scripts-3.4/ -> build/lib/
+    os.path.join(_parent_dir, "lib"),
+    # installed in non-default path: bin/ -> lib/python3.4/site-packages/
+    os.path.join(_parent_dir, "lib",
                  "python%d.%d" % (sys.version_info[0], sys.version_info[1]),
                  "site-packages")
-if os.path.exists(_module_path) and _module_path not in sys.path:
+]:
+    if os.path.exists(_module_path) and _module_path not in sys.path:
         sys.path = [_module_path] + sys.path
 
 import binwalk