add help(), help_targets(), help_ips() functions for help messages

[Risto97]

This pull request adds functions:

• help_targets() - Prints all targets and their description in the build
• help_ips() - Prints all IPs and their descriptions.
• help() - Prints both targets and IPs help messages

To make this useful, all the tool functions should set DESCRIPTION property on the targets it creates for 2nd column to be useful. (mentioned #64 )
add_ip() should also have DESCRIPTION optional argument (mentioned #62 ) in order to have useful information.

These functions can be added only once per build flow.
It is recommended to add it at the end of the CMakeLists.txt file.

In order to ensure it is the only help() call, all levels of hierarchy should have a "top level guard" around the call:

if(PROJECT_IS_TOP_LEVEL)
    help()
endif()

[Risto97]

resolve #63

[Risto97]

Resolve #61