Add support for mesanet ethernet UDP boards 7i80 and 7i76E

driver works under rt-preempt and partially under xenomai.
this driver don't use rtnet, but standard system sockets. Finally it will use rtnet layer of xenomai (and maybe rtai)
but for easy testing, driver development and required complex changes of hostmot2 driver it will be now in this state.

syntax:
loadrt hm2_eth debug=0 board_ip="192.168.1.121" board_mac="00:60:1b:10:40:00" config="sserial_port_0=x0xxxxxx num_encoders=1 num_pwmgens=0 num_stepgens=5"

board_mac is needed for adding static arp entry in system
debug=1 will enable logging every data transmition to/from board

under rt-preempt encoder and sserial was tested with 16h test and worked ok
under xenomai there are huge spiked which hang up sserial communication (wip)

Signed-off-by: Michael Geszkiewicz <micges@wp.pl>
hm2_eth: add more data to debug output

Signed-off-by: Michael Geszkiewicz <micges@wp.pl>
hm2_eth: add retry with timeout to enqueue_read() to make sure that all requested data will be readed

Signed-off-by: Michael Geszkiewicz <micges@wp.pl>