system-linux: add support for configurable GRO option

system-linux: add support for configurable GRO option

Add support for configurable GRO option. Some device doesn't have HW
Checksum support and may suffer from performance regression by using
GRO. Disabling GRO restore the original performance and make the device
usable again.

The option can be configured by adding the config for the device in the
network config. Example:

config device
	option name 'eth0'
	option gro '0'

The option can also be configured by adding the config to the
board.json.

Notice that a new "kind" of settings are introduced
"system_if_apply_settings_after_up". Option set in this function will be
executed AFTER the interface is UP. This is needed as some option
(example GRO) needs to be applied after the interface is UP and applying
them before results in error in ioctl.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>