The DHCPEnabled property within the EthernetInterfaces YAML is not a boolean

[Howitzer105mm]

While implementing the Redfish DHCP controls I found that the DHCPEnabled property is incapable of supporting the Redfish feature set.

The DHCPEnabled property is described as:

name: DHCPEnabled
type: boolean
Redfish allows, and systemd allows, greater control than just Statically assigned or Both Dynamically assigned.
Systemd tests for the following values:
"any boolean false (i.e. 0, false, f, no, n, none, or off)": static addressing only
"any boolean true (i.e. 1, true, t, yes, y, on)": enable DHCP for IPv4 and IPv6
"ipv4" or "v4": DHCP for IPv4 only, static IPv6
"ipv6" or "v6": DHCP for IPv6 only, static IPv4
"both": enable DHCP for IPv4 and IPv6

Systemd reads these values from the 00-bmc-ethx.network files [Network] section:
[Match]
Name=eth1
[Network]
LinkLocalAddressing=yes
IPv6AcceptRA=true
DHCP=false

To avoid having to make a change to both phosphor-network and systemd, it is preferred to modify the EthernetInterface property for DHCPEnabled to be an enum type, allowing it to accept a subset of the values currently accepted by systemd. The subset should be: false, true, ipv4, ipv6. I have manually updated the 00-bmc-ethx.network file with these four values and have determined that the ethernet interface does perform the correct DHCP behavior for each.

I plan to make this change and to submit it for review.