fix releasetools for non-linux architectures

The ota and img building scripts contained some hardcoded 'linux-x86'
paths.  Remove and replace with a slightly redefined -p option.
Modify Makefile to pass correct -p when building.
diff --git a/tools/releasetools/sign_target_files_apks b/tools/releasetools/sign_target_files_apks
index bc04956..6dd8ede 100755
--- a/tools/releasetools/sign_target_files_apks
+++ b/tools/releasetools/sign_target_files_apks
@@ -278,7 +278,9 @@
   # recovery uses a version of the key that has been slightly
   # predigested (by DumpPublicKey.java) and put in res/keys.
 
-  p = common.Run(["java", "-jar", OPTIONS.dumpkey_jar] + mapped_keys,
+  p = common.Run(["java", "-jar",
+                  os.path.join(OPTIONS.search_path, "framework", "dumpkey.jar")]
+                 + mapped_keys,
                  stdout=subprocess.PIPE)
   data, _ = p.communicate()
   if p.returncode != 0: