uboot_arm: board has changed name from rpi_b => rpi
uboot_arm: board has changed name from rpi_b => rpi

Happened in this commit:
http://git.denx.de/?p=u-boot/u-boot-arm.git;a=commit;h=9316e14400c255e02f66bfb6d1e3d1dce3daf86a

notro/rpi-build-stdlib