For boards configured with an RW B side, produce a hex file for the B side.
For boards configured with an RW B side, produce a hex file for the B side.

If a board configuration produces an RW image for its B side, it makes
sense that requesting the "hex" target would produce a hex file for the B
side image as well as the A side image.

BUG=none
BRANCH=none
TEST=make -j buildall

Signed-off-by: Carl Hamilton <32ebda5be60bff17e355b43d7795cb4f4fd6ece9@chromium.org>
Change-Id: Ib4ff1e41272ebf199c6361da4b572a43d403a003
Reviewed-on: https://chromium-review.googlesource.com/1369686
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec