pkg-generic: prevent _SITE URLs with a trailing slash

pkg-generic: prevent _SITE URLs with a trailing slash

A trailing slash in FOO_SITE is useless, since Buildroot automatically adds
a slash between FOO_SITE and the filename as appropriate.

Moreover it is potentially harmful, which led to introducing a workaround
to strip them:

    commit 1cbffbd015106ea90fe49e27433375769dc1035b
    Author: Shawn J. Goff <shawn7400@gmail.com>
    Date:   Fri Apr 12 09:40:30 2013 +0000

        eliminate double slashes caused by FOO_SITE ending in a slash

        When a FOO_SITE variable ends in a slash and gets joined with a
        FOO_SOURCE variable like $(FOO_SITE)/$(FOO_SOURCE), the resulting URI
        has a double slash. While double-slashes are fine in unix paths, they
        are reserved in URIs - the part following '//' must be an authority.

So let's ban trailing slashes entirely. They have all been removed in
a 7b0e757fb85fd, now add a check to error out loudly in case a new one
is added.

Example commands to test this check:

  $ make busybox-dirclean busybox-source
  rm -Rf /home/murray/devel/buildroot/output/build/busybox-1.23.2
  busybox-1.23.2.tar.bz2: OK (md5: 7925683d7dd105aabe9b6b618d48cc73)
  busybox-1.23.2.tar.bz2: OK (sha1: 7f37193cb249f27630e0b2a2c6c9bbb7b1d24c16)
  $
  $ make BUSYBOX_SITE=http://www.busybox.net/downloads/ busybox-dirclean busybox-source
  rm -Rf /home/murray/devel/buildroot/output/build/busybox-1.23.2
  BUSYBOX_SITE (http://www.busybox.net/downloads/) cannot have a trailing slash
  make[1]: *** [/home/murray/devel/buildroot/output/build/busybox-1.23.2/.stamp_downloaded] Error 1
  make: *** [_all] Error 2
  $

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>