Fix joining of DESTDIR and group root

Fix up the path joining logic:

 *  For obvious reasons, we panic if group's root is empty.

 *  DESTDIR can be empty, that's normal for installs such as when MKit
    installs its own dogfood.

 *  Root (which sometimes starts with PREFIX) may or may not start with
    slash; relative root such as that is also OK for installs like MKit
    or TFKit.

 *  Furthermore, DESTDIR may or may not end with slash; we have to address
    that and avoid joining the names.

src/include/deploy.sh

@@ -69,8 +69,13 @@ get_root() {
     #
     local grp="$1"
     local root=$(ini 1value "roots:$grp")
-    test -n "$root" || die "missing in config.ini: roots:$grp"
-    echo "$(ini 1value ENV:DESTDIR)$root"
+    local destdir=$(ini 1value ENV:DESTDIR)
+    destdir=${destdir%/}
+    case $destdir:$root in
+        *:)     die "missing in config.ini: roots:$grp" ;;
+        :*)     echo "$root" ;;
+        *:*)    echo "$destdir/$root" ;;
+    esac
 }
 
 install() {