support/download: add download wrapper

support/download: add download wrapper

The download wrapper is responsible for ensuring the atomicity
of saving into $(BR2_DL_DIR).

It calls the appropriate download helper, telling it to save the
downloaded content to a temporary file in $(BUILD_DIR) (so it does
not clutter $(BR2_DL_DIR) with partial, failed downloads.

Then, only if the download helper was successful, does the wrapper
save the downloaded content to the final location, yet still in a
temporary file, and finally atomically renames it to the final output
file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>