boards/raspberrypi: update instructions for Raspberry Pi 2

The instructions for the Raspberry Pi 2 (aka model B2) are the same as
the ones for the Raspberry Pi, so:
  - state so in the existing readme
  - create a symlink raspberrypi2 -> raspberrypi

Also fix typo in spelling of "Raspberry Pi" (two words, not one).

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