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.

(From OE-Core rev: ff2ad51b801fd62e2abbc573ba2c9ee8fdc7e012)

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

meta/classes/populate_sdk_ext.bbclass

@@ -364,6 +364,9 @@ python copy_buildsystem () {
             # Hide the config information from bitbake output (since it's fixed within the SDK)
             f.write('BUILDCFG_HEADER = ""\n\n')
 
+            # Write METADATA_REVISION
+            f.write('METADATA_REVISION = "%s"\n\n' % d.getVar('METADATA_REVISION'))
+
             f.write('# Provide a flag to indicate we are in the EXT_SDK Context\n')
             f.write('WITHIN_EXT_SDK = "1"\n\n')