GNRC MAC protocols: port to gnrc_mac module and add tutorials.

[zhuoshuguo]

This issue is mainly about a list of future works on GNRC MAC protocols and the related modules.

So far, we have LWMAC protocol in RIOT, and will soon have GoMacH protocol. To make the codes more compact and enhance code re-usage and also remove the timer-source rely on RTT, a plan is to port both LWMAC and GoMacH to the gnrc_mac module. Also, we need some more detailed descriptions/instructions about the gnrc_mac module and the related MAC protocols, about how to use and configure them. Finally, we would like to provide a tutorial task about how to configure MAC protocols in RIOT.

TODOs

• Merge GoMacH protocol, #5816;

• Remove "{" in case statements (in LWMAC and GoMacH) to meet the standard coding conventions, as suggested in gnrc_lwmac: enable radio duty-cycle printing. #8942 .

• Adapt the GoMacH's WP extension scheme to be based on RX-start event;

• Use double wake-up for GoMacH to tackle the radio's ill-reception problem (on iotlab-m3);

• Finish and merge the timeout module of gnrc_mac, gnrc_mac: add timeout module. #5949 ;

• Port both GoMacH and LWMAC to gnrc_mac, e.g., utilizing gnrc_mac's timeout module for replacing GoMacH's (LWMAC's) own timeout;

• Removing the rely on RTT for GoMacH and LWMAC;

• Provide a detailed description and tutorial document for gnrc_mac, which introduces its components and usages;

• Provide a MAC shell command to show achieved radio duty-cycle; gnrc_gomach: add duty-cycle display shell command. #8797 gnrc_lwmac: enable radio duty-cycle printing. #8942

• Change LWMAC's radio duty-cycle recording to be based on xtimer but not RTT.

• Provide a MAC shell command to adapt internal parameters, e.g., change the working sub-channel of GoMacH.

• Provide a MAC tutorial task in RIOT's tutorials: https://github.com/RIOT-OS/Tutorials , which introduces how to configure key parameters for obtaining targeted features like low duty-cycle (channel check rate) and shorter packet transmission delay, etc.

Enhancement

• Provide an option to enable the broadcast scheme of GoMacH to be a receiver-initiate based scheme.