wpa_supplicant: Initial connection speedup

wpa_supplicant: Initial connection speedup

Add initial_freq_list to wpa_supplicant configuration. This option
allows wpa_supplicant to scan a smaller list of frequencies when it
starts. This in turn allows for a faster connection to an already known
network. This limit applies only for the initial scan operation and does
not restrict other channels from being used in consecutive scans.

Tests have shown this to reduce the amount of time for connecting to a
network from roughly 3 seconds to roughly 0.1 second.

Signed-off-by: Frederik Juul <frederik.juul@3shape.com>