raspberrypi2: switch Linux kernel to rpi-3.18.y

raspberrypi2: switch Linux kernel to rpi-3.18.y

This changes the kernel used on the Raspberry Pi 2 from the 3.19 branch
back to the 3.18 branch. This provides a couple of advantages:

  1. mmc0 works again. Floris Bos found out that this was due to using
     the precompiled DTB files from rpi-firmware. These DTBs were built
     using 3.18.
  2. The rpi-3.18.y branch is not regularly rebased like rpi-3.19.y
     according to popcornmix. See
     https://github.com/raspberrypi/linux/issues/915.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Reviewed-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>