periph.cpuid: Add central documentation for CPUID_ID_LEN
auto_init: use full hash to configure radio address

Since the 4-byte CPU ID on native is in most cases generated from the 2-byte
PID of the native process, choosing the lower half of the hash of the CPU ID to
set the radio address led to always setting it to the hash of 0 in this
cases. This changes it to use both halves of the hash and taking the
modulus so this corner cases can be avoided.
auto_init: reduce scope of iface
Merge pull request #1814 from authmillenon/auto-init-addr-fix

auto_init: use full hash to configure radio address
Merge pull request #1821 from authmillenon/cpuid-doc

periph.cpuid: Add central documentation for CPUID_ID_LEN