p54: Support boottime in scan results

This patch fixes an issue with wpa_supplicant dropping all scan
results because their where considered to be "too old" (e.g.:
"skip - scan result not recent enough (121056.086325 seconds too old)")
which is very weird because this looks like that the scan results have
been received before a scan started. This is due to the inaccuracy of
the default timing mechanism for calculating the BSS entry age.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>