- support range based for loops
 - add remove() method to remove a node
 - add clear() to empty entire list and delete nodes
 - add empty() helper

Code style fix

Most of the systemcmds tests run in the posix build. The UART tests
fail for me as I do not have a UART connected.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

Modified code to use OS independent header file for config settings.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>