powerpc: derive COMMAND_LINE_SIZE from asm-generic

The default COMMAND_LINE_SIZE in asm-generic is 512, so the
net effect of this change is nil, aside from the cleanup
factor.  See also commit 2b74b856

.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>