support/run-tests: export download dir

support/run-tests: export download dir

Currently, the download directory, when specified with the -d option, is
only used to store the files downloaded by the testing infra, not those
downloaded by Buildroot.

So, we end up with this situation:

    BR2_DL_DIR  | -d DIR   | test downloads   | BR downloads
    ------------+----------+------------------+--------------
    unset       | unset    | [error]          | [error]
    unset       | set      | in $(DIR)        | in $(TOP_DIR)/dl
    set         | unset    | in $(BR2_DL_DIR) | in $(BR2_DL_DIR)
    set         | set      | in $(DIR)        | in $(BR2_DL_DIR)

This is not very consistent.

We change the behaviour so that the value of -d always takes precedence,
and is used by Buildroot as well, giving this new behaviour:

    BR2_DL_DIR  | -d DIR   | test downloads   | BR downloads
    ------------+----------+------------------+--------------
    unset       | unset    | [error]          | [error]
    unset       | set      | in $(DIR)        | in $(DIR)
    set         | unset    | in $(BR2_DL_DIR) | in $(BR2_DL_DIR)
    set         | set      | in $(DIR)        | in $(DIR)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>