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>

board/raspberrypi/readme.txt

@@ -17,11 +17,11 @@ How to build it
 Configure Buildroot
 -------------------
 
-The rpi_defconfig configuration is a minimal configuration with all that
-is required to bring the Raspberry Pi. You should base your work on this
-defconfig:
+The raspberrypi_defconfig configuration is a minimal configuration with
+all that is required to bring the Raspberry Pi. You should base your
+work on this defconfig:
 
-  $ make rpi_defconfig
+  $ make raspberrypi_defconfig
 
 If you want to use a persistent rootfs, skip to "Build the rootfs", below.