externalsrc: fix task dependency for do_populate_lic

externalsrc: fix task dependency for do_populate_lic

do_populate_lic dependencies are defined inside license.bbclass such as:

  addtask populate_lic after do_patch before do_build

but externalsrc deletes the do_patch task, so the only dependency left for
do_populate_lic is "before do_build"

On a devtool context, when doing devtool modify, sources are extracted inside
build/workspace/sources/${BPN}/ and local files inside
build/workspace/sources/${BPN}/oe-local-files

When building the recipe after a devtool modify, do_unpack is called again to
unpack (possibly modified) local files from
build/workspace/sources/${BPN}/oe-local-files into ${WORKDIR}.

Since the only left dependency for do_populate_lic is do_build, the
do_populate_lic can be called BEFORE do_unpack. Most of the time this is not a
problem, because license files are generally located inside ${S}, which
corresponds to build/workspace/sources/${BPN} (and is already unpacked),
but this can lead to an issue if recipe sets LIC_FILES_CHKSUM to look for
files in ${WORKDIR} (example from init-ifupdown_1.0.bb):

  LIC_FILES_CHKSUM = "file://${WORKDIR}/copyright;md5=3dd6192d306f582dee7687da3d8748ab"

So devtool modify init-ifupdown && bitbake init-ifupdown gives the following
error:

  WARNING: init-ifupdown-1.0-r0 do_populate_lic: Could not copy license file <...>/build/tmp/work/qemux86_64-poky-linux/init-ifupdown/1.0/copyright to <...>/build/tmp/work/qemux86_64-poky-linux/init-ifupdown/1.0/license-destdir/qemux86_64/init-ifupdown/copyright: [Errno 2] No such file or directory: '<...>/build/tmp/work/qemux86_64-poky-linux/init-ifupdown/1.0/copyright'
  ERROR: init-ifupdown-1.0-r0 do_populate_lic: QA Issue: init-ifupdown: LIC_FILES_CHKSUM points to an invalid file: <...>/build/tmp/work/qemux86_64-poky-linux/init-ifupdown/1.0/copyright [license-checksum]
  ERROR: init-ifupdown-1.0-r0 do_populate_lic: Fatal QA errors were found, failing task.
  ERROR: Logfile of failure stored in: <...>/build/tmp/work/qemux86_64-poky-linux/init-ifupdown/1.0/temp/log.do_populate_lic.838584
  ERROR: Task (<...>/poky/meta/recipes-core/init-ifupdown/init-ifupdown_1.0.bb:do_populate_lic) failed with exit code '1'

Fix this by forcing the do_populate_lic task to run after do_unpack

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>