Allow for zero cipher length in AES-CCM

- lifted restriction on zero cipher length for AES-CCM
- added test vector to exercise this condition in LibTestApp
- added flushing on MB manager at the end of AES-CCM and AES-CMAC tests
  (LibTestApp)
- fixed potential buffer overrun issue (MB internal) in AES-CCM with non-zero AAD

Change-Id: I82821dc087b74b91e5cb405611abaa2254c73dff