dist/prepare: add log() function, more verbosity, separate sections
dist/prepare: $OBJDIR will never end with / if $out is set

If $out is set, then OBJDIR is unconditionally set from $ARCHOBJ/
$DISTOBJ/$HOSTOBJ, which are set from uname/os-release/hostname resp.
dist/prepare: use a more straightforward "is empty" check
dist/prepare: -v: add option reminders, align columns
dist/prepare: mkdir $OBJDIR before trying to symlink

Some arches (e.g. MinGW) don't like dangling symlinks. (Specifically,
MinGW's `ln` tries to just copy the directory instead of making a link.)