ll: Parametrize advertise interval

This patch adds a parameter to ll_advertise_start() API to specify the
advertise interval.
ll: Parametrize advertise channels

This patch adds a parameter to ll_advertise_sgtart() API to specify
which advertise channels should be used.
bci: Pack advertise parameters into a struct
bci: Add bci_get_advertising_params() API
bci: Add bci_set_advertising_params() API