Fix PREFIX check

Turns out that when DESTDIR is used, the PREFIX dir almost never exists
(such as if DESTDIR points to a build root).

Before DESTDIR support was added, mkit would rely on PREFIX itself.
Now, checks will be done on DESTDIR itself, which must be set internally
in mkit.ini or inherited.  PREFIX is actually considered only inside
mkit.ini itself.

src/include/deploy.sh

 
 check_env() {
     #
-    # Check that environment variables have been set properly
+    # Check that critical variables are set correctly
     #
-    PREFIX="$(readlink -m "$PREFIX")"
-    test -d "$PREFIX" || die "PREFIX points to non-existent directory: $PREFIX"
+    test -n "$destdir" || die "could not determine DESTDIR"
+    test -d "$destdir" || die "DESTDIR: no such directory: $destdir"
 }
 
 deploy_item() {
     #
     # Install product
     #
-    check_env
     local dst group mode src
     local destdir=$(get_destdir)
+    check_env
     ini values "lists:group" \
       | while read group;
         do
     #
     # Uninstall product
     #
-    check_env
     local dst group src
     local destdir=$(get_destdir)
+    check_env
     ini values "lists:group" \
       | while read group;
         do