[FIX] Fix compiler warning in Linux kernel module

Change-Id: Ic3fe79dd61f6d03bcc8dd0ffb1c1c4289610b870
[FIX] Fix potential NULL pointer access in process image functions

Clean up process image module to avoid potential NULL pointer
accesses.
- Check for initialized stack in all process image API functions
- Disable process image allocation if size is 0
- Clean up freeing of process image
- Update function documentation

Change-Id: Ibc309b873b9ffaa4266575b5747578fc495371a7
[FEATURE] Add an Alternative SDO stack

 - Alternative SDO stack for openCONFORMANCE

   For testing the SDO command and sequence layer
   openCONFORMANCE uses an alternative SDO stack.

   To prevent changes in the existing function calls
   an SDO stack manager was introduced. The SDO manager
   forwards the function calls to the stack defined in
   the api init parameters.

   Furthermore functions for testing the SDO command and
   sequence layer were implemented.

Change-Id: I49e5ad2c546b124af7769fa81f5eb158c6261b05
[FIX] Fix compiler warnings in PDO module

For architectures on which the macros OPLK_DCACHE_INVALIDATE()
and OPLK_DCACHE_FLUSH() are set to "dummies", the compiler
produces warnings about unused parameters. To avoid this warning,
the parameters are referenced with the UNUSED_PARAMETER() macro.

Change-Id: I528be9f5663f736b03ce35e080db3e5d3594aa09
[FIX] Fix compilation error in system-linux.c

Change-Id: Iba28c2a7df9a09419b408fada3beaea91f4e97d4