libflashrom: Implicitly assume HOST_FIRMWARE target

`-p host` is the only case used by mosys and so implicitly assume
it instead of passing around configuration parameters for only one
case.

BUG=b:170914222,b:171603352
TEST=builds and then verified on a banjo.

Change-Id: I85a6abf10bc862e2da44f564c998d5a987f8bdee
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/2500802
Commit-Queue: Edward O'Callaghan <quasisec@chromium.org>
Tested-by: Edward O'Callaghan <quasisec@chromium.org>
Auto-Submit: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
4 files changed