sandbox: Use defconfig to enable features

Stop using the sandbox arch Kconfig to override defaults for config
options. This is a bit of abuse and may be causing build problems.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index b333b61..8e0c4cd 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -1,4 +1,7 @@
 CONFIG_DM_USB=y
+CONFIG_NET=y
+CONFIG_NETDEVICES=y
+CONFIG_DM_ETH=y
 CONFIG_PCI=y
 CONFIG_SYS_VSNPRINTF=y
 CONFIG_DEFAULT_DEVICE_TREE="sandbox"
@@ -35,3 +38,4 @@
 CONFIG_DM_RTC=y
 CONFIG_ERRNO_STR=y
 CONFIG_CMD_UT_TIME=y
+CONFIG_DM_TEST=y