kas-container: Add support for menu plugin

For this script, this is almost straightforward - except that we need to
extract the static KAS_BUILD_SYSTEM config setting from the selected
Kconfig file so that the correct container image and mode is chosen.

Two new dependencies need to be added to the container image. While
python3-newt can come from Debian, kconfiglib only exists as Python
package. To make sure we are not pulling any other packages via pip,
install kconfiglib upfront. It has no own dependencies, thus can use
--no-deps as well.

Finally, the container-entrypoint needs to be updated to make it aware
of the new plugin.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

container-entrypoint

@@ -40,7 +40,7 @@ fi
 
 if [ -n "$1" ]; then
 	case "$1" in
-	build|checkout|for-all-repos|shell|-*)
+	build|checkout|for-all-repos|menu|shell|-*)
 		exec $GOSU kas "$@"
 		;;
 	*)