u-boot: add 2010.06 release

A few minor modifications of u-boot.mk is needed to support 2010.06:

- U-Boot now uses ARCH=powerpc like the kernel (instead of ARCH=ppc)
- Library files have moved, so adjust the target mkimage/fw_printenv rules
  (still compatible with older versions)

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>