releasetools: Make additional modules Python 3 compatible.

Bug: 131631303
Test: `python -m unittest test_sign_target_files_apks`
Test: `python3 -m unittest test_sign_target_files_apks`
Test: `python -m unittest test_add_img_to_target_files`
Test: `python3 -m unittest test_add_img_to_target_files`
Test: `python -m unittest test_ota_from_target_files`
Test: `python3 -m unittest test_ota_from_target_files`
Test: `python -m unittest test_validate_target_files`
Test: `python3 -m unittest test_validate_target_files`
Test: Run `python3 ota_from_target_files.py` to generate an OTA.
Test: Run `python3 sign_target_files_apks.py` to sign a target_files.
Change-Id: I56b45bbcbf7aa83e690785a9640c0212e45d12d8
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 4bebef5..f8cb7e9 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -54,7 +54,9 @@
       base_search_path = os.path.join(base_out_path,
                                       os.path.basename(os.getcwd()))
 
+    # Python >= 3.3 returns 'linux', whereas Python 2.7 gives 'linux2'.
     platform_search_path = {
+        "linux": os.path.join(base_search_path, "host/linux-x86"),
         "linux2": os.path.join(base_search_path, "host/linux-x86"),
         "darwin": os.path.join(base_search_path, "host/darwin-x86"),
     }