poplar: use random mac address

Set CONFIG_NET_RANDOM_ETHADDR=y, which sets random eth address in
case there is no configuration provided neither in CONFIG_ETHADDR
nor in "ethaddr" env variable.

This fixes the problem:
poplar# dhcp
Error: ethernet@9841000 address not set.

Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
diff --git a/configs/poplar_defconfig b/configs/poplar_defconfig
index f0ab231..b6e0c31 100644
--- a/configs/poplar_defconfig
+++ b/configs/poplar_defconfig
@@ -22,6 +22,7 @@
 # CONFIG_ISO_PARTITION is not set
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x20000000
 CONFIG_FASTBOOT_BUF_SIZE=0x10000000