configs/raspberrypi_dt: update and fix our defconfig

Lock to 3.18 headers; add comment about installing DTBs;
bump kernel sha1.

Reported-by: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>