Pass MAC address from uboot to kernel, makes netboot over NFS work properly

This patch makes uboot netbooting work for my system, hooray! 
Before that, the kernel driver smsc95xx could not determine the MAC address of the RPI, altough u-boot already knew the correct one. So I implemented the same behaviour as presumably start.elf does when directly booting kernels, i.e. appending something like "smsc95xx.macaddr=B8:27:EB:59:EB:25" to the kernel command line, see here: http://www.raspberrypi.org/forums/viewtopic.php?t=33252

I implemented first the feature that cmd_pxe does not simply overwrite the bootargs environment variable, but prepends the contents if bootargs is already set. Then I changed the pxe boot script to set bootargs to smsc95xx.macaddr=${usbethaddr}

I tested it here, and it works, but I don't know if this is really the proper way to fix it. Comments welcome!