Allow a single DHCP configuration via the system configuration submenu

This patch allows the setup of simple a single interface to be
automatically brought up and configured via DHCP on system startup.

The interface name can be set via a configuration option. This patch
does not support systemd-networkd, any complex network configuration should
be done via overlay of /etc/network/interfaces or the relevant networkd
configuration file

[Peter: rename to BR2_SYSTEM_DHCP, tweak help text & implementation]
Signed-off-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

system/Config.in

@@ -326,6 +326,26 @@ config BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW
 
 endif # BR2_ROOTFS_SKELETON_DEFAULT
 
+
+config BR2_SYSTEM_DHCP
+	string "Network interface to configure through DHCP"
+	default ""
+	depends on !BR2_PACKAGE_SYSTEMD_NETWORKD && (BR2_PACKAGE_BUSYBOX || BR2_PACKAGE_IFUPDOWN)
+	help
+	  Enter here the name of the network interface (E.G. eth0) to
+	  automatically configure through DHCP at bootup.
+
+	  If left empty, no automatic DHCP requests will take place.
+
+	  For more complicated network setups use an overlay to overwrite
+	  /etc/network/interfaces or add a networkd configuration file.
+
+comment "automatic network configuration via DHCP is not compatible with networkd"
+	depends on BR2_PACKAGE_SYSTEMD_NETWORKD
+
+comment "automatic network configuration via DHCP needs ifupdown or busybox"
+	depends on !(BR2_PACKAGE_BUSYBOX || BR2_PACKAGE_IFUPDOWN)
+
 config BR2_TARGET_TZ_INFO
 	bool "Install timezone info"
 	# No timezone for musl; only for uClibc or (e)glibc.

system/skeleton/etc/network/interfaces

@@ -1,4 +0,0 @@
-# Configure Loopback
-auto lo
-iface lo inet loopback
-

system/system.mk

@@ -38,6 +38,35 @@ ifneq ($(TARGET_GENERIC_ROOT_PASSWD),)
 TARGETS += host-mkpasswd
 endif
 
+define SET_NETWORK_LOCALHOST
+	( \
+		echo "# interface file auto-generated by buildroot"; \
+		echo ;                                               \
+		echo "auto lo";                                      \
+		echo "iface lo inet loopback";                       \
+	) > $(TARGET_DIR)/etc/network/interfaces
+endef
+
+NETWORK_DHCP_IFACE = $(call qstrip,$(BR2_SYSTEM_DHCP))
+
+ifneq ($(NETWORK_DHCP_IFACE),)
+define SET_NETWORK_DHCP
+	( \
+		echo ;                                               \
+		echo "auto $(NETWORK_DHCP_IFACE)";                   \
+		echo "iface $(NETWORK_DHCP_IFACE) inet dhcp";        \
+	) >> $(TARGET_DIR)/etc/network/interfaces
+endef
+endif
+
+define SET_NETWORK
+	mkdir -p $(TARGET_DIR)/etc/network/
+	$(SET_NETWORK_LOCALHOST)
+	$(SET_NETWORK_DHCP)
+endef
+
+TARGET_FINALIZE_HOOKS += SET_NETWORK
+
 ifeq ($(BR2_ROOTFS_SKELETON_DEFAULT),y)
 
 define SYSTEM_ROOT_PASSWD