Make it possible to override the default mipsel sysroot.

Also, update the default mipsel sysroot to match the work in issue 12315003.

Review URL: https://chromiumcodereview.appspot.com/12387011

git-svn-id: http://src.chromium.org/svn/trunk/src/build@186251 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -617,7 +617,7 @@
         }], # OS=="linux" and target_arch=="arm" and chromeos==0
 
         ['target_arch=="mipsel"', {
-          'sysroot': '<!(cd <(DEPTH) && pwd -P)/native_client/toolchain/linux_mips-trusted/sysroot',
+          'sysroot%': '<!(cd <(DEPTH) && pwd -P)/mipsel-sysroot/sysroot',
         }],
       ],