boards/raspberrypi: update readme with RPi-2 specifics

The RPi-2 has its own DTB, so document it in the list of generated
files.

Also, not all DTBs need to be present on the boot partition, so add a
comment that identifies which should be copied for each model.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>