dev/ep_pfilter: Updated filter rules for wr-nic (with etherbone) project.

- Added CONFIG_NIC_PFILTER in Kconfig to decide which filter rules will be used.
- Updated the dev/ep_pfilter.c file by using CONFIG_NIC_PFILTER. This is done because packet filter does
not work correctly if we write all rules in the CONFIG_ETHERBONE section.
- We do not use the DROP instruction because all other packets go to NIC core.

Note: filter rules to the wr-nic do not have the DROP instruction because all traffic
does not go to the LM32/Etherbone is re-direct to the NIC by default.
cmd_refresh: A new CLI command added to change update time period of gui and stat statistics

	- Add new command "refresh" to change update period of gui and stat
	- Delete "stat cont" command
	- Update "stat" command to work as older "stat cont" one
	- Update the wrc_main.c file to check if the log messages must be generated one time

Note: If you set period to 0, the log messages are only generated one time.
doc: Updated wrpc.in document

	+ Added information about the refresh command
	+ Deleted references about "stat cont" command ("stat" command works as it now)
	+ Modified the stat examples of "Running the Core" section