populate_sdk_ext: record METADATA_REVISION

As we delete the .git/ directory, it's impossible to get METADATA_REVISION
inside eSDK. Because of this, we meet the following warning when installing eSDK.

  WARNING: The base-files:do_install sig is computed to be 16b9d96148d45de183cc94667aae016ec7d102d48255456381e718cd4bbd0aa0, \
  but the sig is locked to 6eb0dcaed504282becee94662481d79264db920dee1f7deda18230133fff8f36 in SIGGEN_LOCKEDSIGS_t-qemux86-64

So we record METADATA_REVISION in eSDK generation time to fix this problem.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>