ci: move imgtool publishing to GH workflows

Disable imgtool publishing on Travis; update scripts to work on both
Travis and GH; add GH workflow for publishing.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
diff --git a/ci/imgtool_install.sh b/ci/imgtool_install.sh
index 94264f6..b8715a4 100755
--- a/ci/imgtool_install.sh
+++ b/ci/imgtool_install.sh
@@ -12,10 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-if [[ $TRAVIS_PULL_REQUEST != "false" || $TRAVIS_BRANCH != "master" ]]; then
-    echo "Either a PR or not \"master\" branch, exiting"
-    exit 0
+if [[ $TRAVIS == "true" ]]; then
+    if [[ $TRAVIS_PULL_REQUEST != "false" || $TRAVIS_BRANCH != "master" ]]; then
+        echo "Either a PR or not \"master\" branch, exiting"
+        exit 0
+    fi
 fi
 
 pip3 install setuptools twine packaging
-pip3 install --pre imgtool
+pip3 install --pre imgtool --no-binary :all: