Mesa Hostmot2 Xenomai4 OOB networking support

[hdiethelm]

This PR adds Xenomai4 EVL out of band networking support for Mesa Hostmot2.

Out of band networking is basically a fast path inside the xenomai real time kernel that enables networking without involving the normal kernel.

Due to some users might want to use Xenomai4 with the standard kernel networking, I decided to create a new component called hm2_eth_evl while hm2_eth behaves exactly like before.

Common code is left in hm2_eth.c and network specific code is moved to hm2_eth_net.c and hm2_eth_net_evl.c.
The linker is used to link hm2_eth_evl and hm2_eth with two different network implementations and the same common code.

A few changes in the existing code where performed:

• Fix a bug in src/rtapi/uspace_xenomai_evl.cc. This resulted in a while(true) error message when the kernel and the userspace library did not match.
• Pass board instead of board->fd to network functions.
• Don't use a cumbersome void pointer casting one line function for SIOCSARP
• The ifname is fetched in init_board() and stored in board, so the install_firewall_perinterface() loop is simplifyed and I can reuse the name inside the evl implementation
• The ip address is stored in board because I need it later
• Print which board is connected, nice if you have more than one board and it throws an error

Things still open:

• I have to set the Xenomai4 C and LD flags to all modules, I did not find a way to only set it for a specific module in the Makefile. It will only change anything if you have libevl installed.
• There are more global symbols. Would it make sense to rename fetch_hwaddr to hm2_fetch_hwaddr for example to avoid conflicts?
• Loading hm2_eth_evl and hm2_eth at the same time would probably create a runtime linker issue which generates undefined behavior.

It's a bit hard to review due to moved code. git diff master:src/hal/drivers/mesa-hostmot2/hm2_eth.c hm2_eth_oob_v5:src/hal/drivers/mesa-hostmot2/hm2_eth_net.c helps.

[hdiethelm]

BTW: I know, rootless xenomai is still todo. I will do that after #4132 is in. It will conflict otherwhise.