clean_header: Run outside of $ANDROID_BUILD_TOP

Enable the use case where we run clean_header.py from outside of
$ANDROID_BUILD_TOP. Previously, this script required the current working
directory to be under $ANDROID_BUILD_TOP. Running it from a different
directory resulted in the following error message:

 clean_header.py: error: Not in android tree pointed at by ANDROID_BUILD_TOP (....)

Change-Id: I48210ea1a0033228a9aaa4124d28247b07cee6d4

libc/kernel/tools/clean_header.py

@@ -152,8 +152,8 @@ if __name__ == "__main__":
         usage()
 
     no_update = True
-    dst_dir = get_kernel_dir()
-    src_dir = get_kernel_headers_original_dir()
+    dst_dir = None
+    src_dir = None
     for opt, arg in optlist:
         if opt == '-u':
             no_update = False
@@ -163,6 +163,15 @@ if __name__ == "__main__":
             src_dir = arg
         elif opt == '-d':
             dst_dir = arg
+    # get_kernel_dir() and get_kernel_headers_original_dir() require the current
+    # working directory to be a direct or indirect subdirectory of
+    # ANDROID_BUILD_TOP.  Otherwise, these functions print an error message and
+    # exit.  Let's allow the user to run this program from an unrelated
+    # directory, if they specify src_dir and dst_dir on the command line.
+    if dst_dir is None:
+      dst_dir = get_kernel_dir()
+    if src_dir is None:
+      src_dir = get_kernel_headers_original_dir()
 
     if len(args) == 0:
         usage()