wpa_supplicant: add support for simple background scan

wpa_supplicant: add support for simple background scan

CONFIG_BGSCAN_SIMPLE is missing from the defconfig included with
wpa_supplicant.  Many programs that depend on wpa_supplicant assume
bgscan is enabled, for example connman will try to set bgscan by default.
It is safe to always enable CONFIG_BGSCAN_SIMPLE since it is only used if
wpa_supplicant is configured to do so.

Signed-off-by: Martin Bark <martin@barkynet.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>