hostname: add 'convertible' chassis type

Add the 'convertible' type to the set of allowed chassis. This applies
to all devices that can be transformed by the user from laptop style to
tablet style.

This does not add any auto-detection, yet. It only makes 'set-chassis'
accept 'convertible' as valid input.

shell-completion/bash/hostnamectl

@@ -52,7 +52,7 @@ _hostnamectl() {
         if [[ -z $verb ]]; then
                 comps=${VERBS[*]}
         elif __contains_word "$verb" ${VERBS[CHASSIS]}; then
-                comps='desktop laptop server tablet handset watch embedded vm container'
+                comps='desktop laptop convertible server tablet handset watch embedded vm container'
         elif __contains_word "$verb" ${VERBS[STANDALONE]} ${VERBS[ICONS]} ${VERBS[NAME]}; then
                 comps=''
         fi