x86: Set a board-size limit for chromebook_link

Set a size limit for this board so that we get a build error if it grows
too much.

Note that the limit is approximately, since it does not include the FDT,
microcode and fdtmap, which can change in size. However this board is
fairly stable, so overflowing this limit will likely result in the image
not fitting in the ROM space available for U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>