Network (Uplink): Implement option "dhcp_client"

[LipkeGu]

This PR implements on "corerouter" the option "dhcp_client" as bool on uplink and sets proto: dhcp. The option is documented and by default disabled and not set to keep the current behavier of the devices.

Example locations/(###my_example_location###.yml):

  - vid: 50
    role: uplink
    dhcp_client: false

Current:

config interface 'uplink'
	option device 'br-uplink'
	option proto 'none'

Result:

config interface 'uplink'
	option device 'br-uplink'
	option proto 'dhcp'

Maybe helpfully in issue: #1045