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/Astar_MID756_defconfig b/configs/Astar_MID756_defconfig
index 2029c1f..f334032 100644
--- a/configs/Astar_MID756_defconfig
+++ b/configs/Astar_MID756_defconfig
@@ -16,5 +16,6 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="CONS_INDEX=5"
 CONFIG_CMD_SETEXPR=y
+CONFIG_CMD_NET=y
 CONFIG_AXP221_DLDO1_VOLT=3300
 CONFIG_AXP221_ALDO1_VOLT=3000