tools/ci: Install imgtool for CI build

This is mainly required for macOS builds which do not rely on a base
Docker image that would contain imgtool already preinstalled.

Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>

tools/ci/cibuild.sh

@@ -63,6 +63,11 @@ function python-tools {
   export PYTHONUSERBASE
   add_path $PYTHONUSERBASE/bin
   pip3 install pexpect
+
+  # MCUboot's tool for image signing and key management
+  if ! command -v imgtool &> /dev/null; then
+    pip3 install imgtool
+  fi
 }
 
 function u-boot-tools {