common/cmd_ethsw: Add generic commands for Ethernet Switches

This patch creates a flexible parser for Ethernet Switch
configurations that should support complex commands.
The parser searches for predefined keywords in the command
and calls the proper function when a match is found.
Also, the parser allows for optional keywords, such as
"port", to apply the command on a port
or on all ports. For now, the defined commands are:
ethsw [port <port_no>] { enable | disable | show }

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>