Remove trailing slash from all package site URLs

The recommended form is without the trailing slash. Buildroot will add a slash
between FOO_SITE and FOO_SOURCE as appropriate.

Reported-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>