getEnvDefault: move to a specific header

Change-Id: I3ffbc27f1c00dfc5ad59882f280aacc0e9e97dac
periodictask: make start() a no-op when called from the callback

Change-Id: I402c0be902ac95dce83ba0301b94a25683c612a8
Reviewed-on: http://gerrit.aldebaran.lan/47025
Reviewed-by: lsouchet <lsouchet@aldebaran-robotics.com>
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Tested-by: pdaouadi <pdaouadi@aldebaran-robotics.com>