docs/manual: centralize recommendations about BR2_DL_DIR

docs/manual: centralize recommendations about BR2_DL_DIR

We currently have two places where we recommend where BR2_DL_DIR
should be set: "Environment variables" and "Location of downloaded
packages". The former recommends setting BR2_DL_DIR in the .config,
the latter kind of endorses using ~/.bashrc.

We prefer suggesting the ~/.bashrc way since it avoids downloading the
same file multiple times, and anyway it's wise to have all the details
in a unique place. So remove the .config suggestion from "Environment
variables" and let it just point to "Location of downloaded packages".

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>