sim: Allow 'large-write' in mcuboot-sys

When collected together as a workspace, the test framework ends up
running all of the configurations on each project.  One configuration
option `large-write` doesn't really mean anything to mcuboot-sys, but
adding support for the feature allows this package to be tested along
with all of the other packages.

Signed-off-by: David Brown <david.brown@linaro.org>