rpi-firmware: Fixup config.txt

The default name of the kernel binary for the RaspberryPi is kernel.img, it's a
only a zImage kernel renamed.
Since Buildroot will build a zImage, the default config uses that should use it
instead of forcing the user to rename a file.

Also, remove the cmdline argument rootfstype, as ext4 is a builtin driver,
there is no need to specify it on the cmdline.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
configs: add RaspberryPi defconfig

This is a default configuration for the RaspberryPi, tested and functionnal.

The kernel used is the one developped by the RaspberryPi foundation as it's not
fully supported currently in mainline kernel.
The configuration used for the kernel is the default bcmrpi.

For more info, please find board/raspberrypi/README.txt

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>