configs/ci20_defconfig: configure eth0 through BR2_SYSTEM_DHCP rather than kernel cmdline

Passing ip=dhcp to the kernel will cause it to try to configure the network
interface using DHCP and wait up to 120s for the interface to detect a link,
slowing down boots without a network cable a lot.

Instead use the "normal" BR2_SYSTEM_DHCP, E.G.  trigger ifup to run the DHCP
client in the background.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/ci20/uboot-env.txt

@@ -1,6 +1,6 @@
 baudrate=115200
 board_mfr=NP
-bootargs=console=ttyS4,115200 console=tty0 mem=256M@0x0 mem=768M@0x30000000 rootwait root=/dev/mmcblk0p1 devtmpfs.mount=1 ip=dhcp
+bootargs=console=ttyS4,115200 console=tty0 mem=256M@0x0 mem=768M@0x30000000 rootwait root=/dev/mmcblk0p1 devtmpfs.mount=1
 bootcmd=run ethargs; ext4load mmc 0:1 0x88000000 /boot/uImage; bootm 0x88000000
 bootdelay=1
 ethargs=env set bootargs ${bootargs}

configs/ci20_defconfig

@@ -7,6 +7,7 @@ BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6_1=y
 
 # system
 BR2_TARGET_GENERIC_GETTY_PORT="ttyS4"
+BR2_SYSTEM_DHCP="eth0"
 BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_4=y
 BR2_ROOTFS_POST_IMAGE_SCRIPT="support/scripts/genimage.sh"