bootctl: add --install-source=auto|image|host

When using --root=/--image= the binaries to install/update will be
picked from the directory/image. Add an option to let the caller
choose.
By default (auto) the image is tried first, and if nothing is found
then the host. The other options allow to strictly try the image
or host and ignore the other.

shell-completion/bash/bootctl

@@ -32,7 +32,7 @@ _bootctl() {
     local cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD-1]}
     local -A OPTS=(
         [STANDALONE]='-h --help -p --print-esp-path -x --print-boot-path --version --no-variables --no-pager --graceful'
-        [ARG]='--esp-path --boot-path --make-machine-id-directory --root --image'
+        [ARG]='--esp-path --boot-path --make-machine-id-directory --root --image --install-source'
     )
 
     if __contains_word "$prev" ${OPTS[ARG]}; then
@@ -52,6 +52,9 @@ _bootctl() {
                 compopt -o nospace
                 comps=$( compgen -A file -- "$cur" )
                 ;;
+            --install-source)
+                comps="image host auto"
+                ;;
         esac
         COMPREPLY=( $(compgen -W '$comps' -- "$cur") )
         return 0