ll: Parametrize advertising channels

This patch adds a parameter to ll_advertise_start() 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
bci: Move advertising data mtu definition to bci.h
bdaddr: Move bdaddr_t definition to bdaddr.h
Split adv_type_t in bci_adv_t and ll_pdu_t

Now the LL layer only knows about advertising PDUs and the BCI layer is
responsible to make the translation between the types.
common: Remove header common.h
assert: Add header containing a static assert macro
bci: Assert statically BCI/LL common symbols
nrf51822: Add README
examples: Update README
Update README