package/dhcpcd: systemv and systemd services conflict with NetworkManager

package/dhcpcd: systemv and systemd services conflict with NetworkManager

When NetworkManager and dhcpcd packages are both enabled, dhcpcd
services and NetworkManager both spawn dhcpcd. This causes the network
port to retrieve an IP address and later lose it a few seconds after
startup.

This patch prevents dhcpcd services from launching dhcpcd if
NetworkManager is enabled.

Signed-off-by: Jim Brennan <jbrennan@impinj.com>
[Thomas: add a comment in the code to explain the seemingly strange
condition.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>