opkg: bump to 0.2.4 and fix build failure

This commit bumps opkg to 0.2.4, and adds a new patch to fix a build
failure apparently caused by the recent bump of the libtool version:

  http://autobuild.buildroot.org/results/5fc/5fc9fa24563213d1ad77e55ab52c8e59bf21055f/



It also refreshes the existing patch, turns it into a Git formatted
patch, and fixes a typo in its title.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>