The CPUDIR variable points to the location of a target's CPU directory.
Currently, it is set to cpu/$CPU. However, using $CPUDIR will allow for
more flexibility in the future. It lays the groundwork for reorganizing
U-Boot's directory structure to support a layout such as:
arch/$ARCH/cpu/$CPU/* (architecture with multiple CPU types)
arch/$ARCH/cpu/* (architecture with one CPU type)
Signed-off-by: Peter Tyser <email@example.com>