ARM: rpi_b: detect board revision

Detect the board revision early during boot, and print the decoded
model name.

Eventually, this information can be used for tasks such as:
- Allowing/preventing USB device mode; some models have a USB device on-
  board so only host mode makes sense. Others connect the SoC directly
  to the USB connector, so device-mode might make sense.
- The on-board USB hub/Ethernet requires different GPIOs to enable it,
  although luckily the default appears to be fine so far.
- The compute module contains an on-board eMMC device, so we could store
  the environment there. Other models use an SD card and so don't support
  saving the environment (unless we store it in a file on the FAT boot
  partition...)

Set $fdtfile based on this information. At present, the mainline Linux
kernel doesn't contain a separate DTB for most models, but I hope that
will change soon.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
ARM: rpi: rename rpi_b to rpi

The U-Boot port runs on a variety of RPi models, not just the B. So,
rename the port to something slightly more generic.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
ARM: rpi: support an environment

Enable ENV_IS_IN_FAT so that the environment can be stored persistently.
It's stored in the FAT partition that the RPi firmware requires. On most
RPis, this is on the SD card (which must be present in order for the
system to boot). On the CM this is on the built-in eMMC device.

Since we now have a persistent environment, there's no need to load
uEnv.txt at boot; we only did that to work around the lack of persistent
environment.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
LOCAL: ARM: rpi_b: add script to chain-load U-Boot over DHCP/TFTP

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>