bgscan: Add global bgscan configuration

bgscan: Add global bgscan configuration

This option can be used to globally configure bgscan parameters
for all the network blocks.

Note that this configuration will not override a network block
specific bgscan settings, but will only be used in case that
the network block does not have a valid bgscan configuration.

Signed-hostap: Haim Dreyfuss <haim.dreyfuss@intel.com>