ARM: rpi_b: query internal MAC address from firmware

The built-in SMSC 95xx chip doesn't know its own MAC address. Instead,
we must query it from the VC firmware; it's probably encoded in fuses
on the BCM2835.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
---
v2: Don't set usbethaddr if it's already set
ARM: rpi_b: enable USB/DHCP/PXE in bootcmd

USB support must be enabled before config_distro_bootcmd.h is included
for bootcmd to include USB-related functionality.

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>