iw: add scheduled scan plans configuration

iw: add scheduled scan plans configuration

Scheduled scan now supports configuring multiple 'scan plans'.
Each scan plan specifies the interval between scans and the number
of iterations to scan. So add an option to configure scan plans
for scheduled scan
At least one scan plan must be configured. Note that all scan plans
but the last one must specify the scan interval and the number of
iterations. The last scan plan must specify only the interval and
will be run infinitely.
The option to set only an interval is kept for backwards
compatibility. However, the new scan plans option and the old interval
optoin are mutually exclusive.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>