pkg-download: extend DOWNLOAD_INNER, add a SOURCE_CHECK macro

As part of moving to a package infrastructure based source-check
implementation, we are going to move away from the global DL_MODE
variable to select the behavior of the DOWNLOAD_INNER macro.

As a preparation to this, this commit makes the DOWNLOAD_INNER macro
take a third argument, which is the action to be done: either DOWNLOAD
or SOURCE_CHECK. For now, the DOWNLOAD macro passes $(DL_MODE) as this
third argument, in order to keep the existing behavior.

In addition, a SOURCE_CHECK macro is added, which calls DOWNLOAD_INNER
with the appropriate action. This macro will be used in the upcoming
package infra based implementation of source-check.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[tested by doing a "make source" on a randpackageconfig]
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>