Summary: 2.5.17 broke initrd on x86. Fix below.

Why: Kai's patch in 2.5.17 to move x86-specific options from
Makefile to arch/i386/boot/Makefile unfortunately lost the fact
that the orginal "#export RAMDISK = -DRAMDISK=512" statement
was commented out. (I suspect a typo.) RAMDISK is obsolete since
1.3.something, and uncommenting it has "interesting" effects
since the ram_size field has a very different meaning now.

The patch below reverts the statement to its pre-2.5.17 state.
Perhaps it should be removed altogether?