core: ensure we use the realpath(3) of DL_DIR

When $(TOPDIR)/dl is a symlink, checking out git submodules can fail,
as reported by Michael in #11086.

To reproduce a similarly-related mis-behaviour:

    $ mkdir -p foo/bar foo/buz
    $ cd foo/bar
    $ ln -s ../buz meh
    $ cd meh
    $ cd ../../foo

The last command should not succeed, because, relative to meh, there is
no ../../foo directory; we would expect it to be ../../../foo, instead.
But since meh is a symlink to a directory, then a relative path from that
symlink is interpreted as relative to the derefrenced directory, i.e.
from buz in this case.

But where this gets even weirder, is that, if the last command is
replaced by:

    $ cd ../../../foo

then it still works, too.

And that is the root of Michael's issue: the dl directory in Buildroot's
TOPDIR is a symlink to a similarly-named directory one directory higher,
which then confuses relative paths, which gets especially and noticeably
bad for git submodules.

Avoid this strangeness, and just use so-called "physical" path, i.e. a
path where all symlinks to directories have been dereferenced.

Fixes: #11086

Reported-by: Michael Nosthoff <posted@heine.so>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Michael Nosthoff <posted@heine.so>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>