selftests: forwarding: Add ethtool_lib.sh

Functions:
1. speeds_arr_get
	The function returns an array of speed values from
        /usr/include/linux/ethtool.h The array looks as follows:
	[10baseT/Half] = 0,
	[10baseT/Full] = 1,
	...

2. ethtool_set:
	params: cmd
	The function runs ethtool by cmd (ethtool -s cmd) and checks if
	there was an error in configuration

3. dev_speeds_get:
	params: dev, with_mode (0 or 1), adver (0 or 1)
	return value: Array of supported/Advertised link modes
	with/without mode

	* Example 1:
	speeds_get swp1 0 0
	return: 1000 10000 40000
	* Example 2:
	speeds_get swp1 1 1
	return: 1000baseKX/Full 10000baseKR/Full 40000baseCR4/Full

4. common_speeds_get:
	params: dev1, dev2, with_mode (0 or 1), adver (0 or 1)
	return value: Array of common speeds of dev1 and dev2

	* Example:
	common_speeds_get swp1 swp2 0 0
	return: 1000 10000
	Assuming that swp1 supports 1000 10000 40000 and swp2 supports
	1000 10000

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>