mac80211_hwsim: Support boottime in scan results

This makes the age information for cfg80211 scan results more accurate
and fixes issues with wpa_supplicant dropping "old" scan results (e.g.,
"wlan0: Own scan request started a scan in 0.000456 seconds") that
looked like would have been received before a scan started due to the
inaccuracy of the default timing mechanism for calculating the BSS entry
age. This makes hwsim test cases significantly more robust to run.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>