docs/manual: do not suggest to create a symlink for shared download directory

Creating a symbolic link for the download directory is really not a
useful suggestion, since we have the much better solution of the
BUILDROOT_DL_DIR environment variable. So emphasize this solution
instead, and remove the suggestion of the symbolic link.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>