UBUNTU: snapcraft.yaml: copy retpoline-extract-one to scripts before build

BugLink: http://bugs.launchpad.net/bugs/1782116

The Ubuntu kernel source code depends on the presence of the
retpoline-extract-one file in the script directory during build (see
scripts/Makefile.build::cmd_ubuntu_retpoline) - such a file lives in the debian
directory and is copied to scripts during the 'debian/rules clean' phase.

Snapcraft is oblivious to the debian details, and the clean target is never
invoked, breaking the normal kernel build (make defconfig; make ...).

To workaround that, before starting the build, make snapcraft do the copy and
fix the build.
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

snapcraft.yaml

@@ -17,6 +17,10 @@ parts:
     kconfigflavour: generic
     kconfigs:
       - CONFIG_DEBUG_INFO=n
+    override-build: |
+      cp debian/scripts/retpoline-extract-one \
+        $SNAPCRAFT_PART_BUILD/scripts/ubuntu-retpoline-extract-one
+      snapcraftctl build
   firmware:
     plugin: nil
     stage-packages: