download: put most of the infra in dl-wrapper

download: put most of the infra in dl-wrapper

The goal here is to simplify the infrastructure by putting most of the
code in the dl-wrapper as it is easier to implement and to read.

Most of the functions were common already, this patch finalizes it by
making the pkg-download.mk pass all the parameters needed to the
dl-wrapper which in turn will pass everything to every backend.

The backend will then cherry-pick what it needs from these arguments
and act accordingly.

It eases the transition to the addition of a sub directory per package
in the DL_DIR, and later on, a git cache.

[Peter: drop ';' in BR_NO_CHECK_HASH_FOR in DOWNLOAD macro and swap cd/rm
	-rf as mentioned by Yann, fix typos]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>