configs: rename rpi_defconfig for consistency

We name all of our boards and defconfig files based on the boards
fullname, not a nickname or a shortname.

'rpi' is short for Raspberry Pi, so name all our Raspberry Pi ressource
with 'raspberrypi' instead of 'rpi'.

This should also help Buildroot-newcomers to recognise Raspberry Pi
related files (defconfig and board doc).

Signed-off-by: Mathieu Benoit <mathieu.benoit@savoirfairelinux.com>
[yann.morin.1998@free.fr: reverse the rename]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ryan Barnett <rjbarnet@rockwellcollins.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>