crossystem: Revise description of sw_wpsw_boot.
crossystem: Revise description of sw_wpsw_boot.

The sw_wpsw_boot was made for some feature that was almost never completed, and
only makes sense on Baytrail platforms. To prevent confusion we should address
that in the crossystem description.

BRANCH=none
BUG=chromium:508269
TEST=make test

Change-Id: I1fbc7a0e9e8c1f8503ae8ae9dfb6e80c8da892e3
Reviewed-on: https://chromium-review.googlesource.com/284425
Tested-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Reviewed-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>
Commit-Queue: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot