bci: Prefix ENABLE and DISABLE definitions with BCI
nrf51822: Remove hardcoded path to NRF51 SDK

Now, the user needs to provides NRF51_SDK_PATH to the build system. He
can pass it via make command line:

	$ make NRF51_SDK_PATH=/path/to/sdk

Or put it inside Makefile.config (recommended way):

	$ echo "NRF51_SDK_PATH=/path/to/sdk" >> Makefile.config
	$ make

NRF51_SDK_PATH must point to the directory that contains `nrf51822`
path.

Currently, blessed only supports the SDK version 4.4.x.
nrf51822: Add support for other Nordic boards

Until now, only old versions of PCA10001 evaluation board were
supported. This patch allows the user to reassign those variables
in order to compile blessed to those boards.

This patch wasn't tested with other nRF51822 boards.
ll: Move advertising length-related definitions to ll.h
examples: Use AD structures in examples
ll: Add ll_set_advertising_params() API
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 maximum length definition to bci.h
bdaddr: Move bdaddr_t definition to bdaddr.h
blessed: 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
blessed: Update README
ll: Add ll_get_advertising_data_buffer() API
bci: Add bci_get_advertising_data_buffer() API