linux{, -headers}: support downloads of v5+

linux{, -headers}: support downloads of v5+

With the arrival of linux v5.0, we need yet another condition to set
_SITE correctly. Instead of continuing this madness, solve the problem
generically: use v2.6 for 2.6.*, and use the number before the first dot
in the other cases.

While we're at it, remove the comment which has been incorrect since
80d7b68167a5c8893e906ace6b5f0b0166336406 (7 years ago).

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Tested-by: Adam Duskett <aduskett@gmail.com>
Reviewed-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>