configs/raspberrypi2: kernel version does not exist anymore

It looks like version 23c76b7 of Raspberry kernel does not exist
anymore:

$ git clone https://github.com/raspberrypi/linux.git


[...]
$ cd linux
$ git co 23c76b7f608e51258c6fca02aebeb5d588583149
fatal: reference is not a tree: 23c76b7f608e51258c6fca02aebeb5d588583149

Replace it by latest version of 3.19 branch.

[Thomas: tweak commit log.]

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>