populate_sdk_ext.bbclass: set METADATA_REVISION with an DISTRO override

* otherwise it ends '<unknown>' inside esdk, because of parsing order:
  # $METADATA_REVISION [3 operations]
  #   set /OE/build/test-D/conf/local.conf:43
  #     "f2da54ef432eac89b0f18eaad68e602b6990b5de"
  #   immediate /OE/build/test-D/layers/poky/meta/classes/metadata_scm.bbclass:9
  #     "${@oe.buildcfg.detect_revision(d)}"
  #   set /OE/build/test-D/layers/poky/meta/classes/metadata_scm.bbclass:10
  #     [vardepvalue] "${METADATA_REVISION}"
  # pre-expansion value:
  #   "<unknown>"
  METADATA_REVISION="<unknown>"

* This causes base-files.do_install and following tasks to have different
  signatures between esdk and the build directory where this esdk was created:

  bitbake-diffsigs {test-D,poky/build-uninative-disabled}/tmp/stamps/qemux86_64-poky-linux/base-files/*do_install*sigdata*
  NOTE: Starting bitbake server...
  basehash changed from 5b6981cf58bfd57d416b0e31611b73a26baae635dd1ac31c08d46f95064c3ffc to dbdce042da4d7813d632b6d1cc87a16f728ad20e55fecbc392830e6acf72babd
  Variable METADATA_REVISION value changed from '<unknown>' to 'f2da54ef432eac89b0f18eaad68e602b6990b5de'

  and an warning from "python3 /OE/build/test-D/ext-sdk-prepare.py" when eSDK is being prepared for use:
  WARNING: The base-files:do_install sig is computed to be 83b9c9a6ef1145baac5a1e0d08814b9156af239c58fc42df95c25a9cd8a7f201,
    but the sig is locked to 3dc22233059075978e5503691e98e79e7cc60db94259dfcd886bca2291c0add7 in SIGGEN_LOCKEDSIGS_t-qemux86-64

[RP: Add commit about why we need the override for future reference]
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
(cherry picked from commit 675ea728)
Signed-off-by: Steve Sakoman <steve@sakoman.com>