net: Move the CMD_NET config to defconfigs

This also selects CONFIG_NET for any CONFIG_CMD_NET board.

Remove the imx default for CONFIG_NET.

This moves the config that was defined by 60296a8 (commands: add more
command entries in Kconfig).

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/M54418TWR_serial_rmii_defconfig b/configs/M54418TWR_serial_rmii_defconfig
index dceaab3..4c8876f 100644
--- a/configs/M54418TWR_serial_rmii_defconfig
+++ b/configs/M54418TWR_serial_rmii_defconfig
@@ -1,3 +1,4 @@
 CONFIG_M68K=y
 CONFIG_TARGET_M54418TWR=y
 CONFIG_SYS_EXTRA_OPTIONS="CF_SBF,SYS_SERIAL_BOOT,SYS_TEXT_BASE=0x47E00000,SYS_INPUT_CLKSRC=50000000"
+CONFIG_CMD_NET=y