libweave: Add command line flag to override ssid

Custom SSID can be useful test client applications.

BUG=none
TEST='FEATURES=test emerge-gizmo buffet'

Change-Id: I2a52543df2d62a35a38e2277bff865a0962069b9
Reviewed-on: https://chromium-review.googlesource.com/286662
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>