stack: Add header file with error codes
nrf51822: Check if log was already initialized in log_init()
log: Change log_print() return type
nrf51822: Return error codes in timer module
nrf51822: Return error codes in radio module
ll: Initial implementation of the Link Layer

This commit creates a basic Link Layer with only one state machine and
supporting only non-connectable undirect advertising.
ll: Change return type of the APIs
blob-advertiser: Implement blob advertisement as an app