powerpc: compile kernel for linkstations optimized for size

Since Linkstations and Kuroboxes often have *very* little memory (as
they are embedded systems), it is desirable to get their kernels
compiled optimized for size.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>