net: Move network rules to drivers/net

The code under drivers/net is related to ethernet networking drivers, in
some fashion or another.  Drop these from the top-level Makefile and
also move the phy rule into drivers/net/Makefile which is where it
belongs.  Make the new rule for drivers/net check for the build-stage
relevant ETH symbol.

Fix up some Kconfig dependencies while we're here to mirror how the
Makefile logic now works.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Introduce ETH, Kconfig dependency changes, am43xx fix]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/net/eth_common.c b/net/eth_common.c
index 58f899a..82d527a 100644
--- a/net/eth_common.c
+++ b/net/eth_common.c
@@ -32,6 +32,7 @@
 void eth_common_init(void)
 {
 	bootstage_mark(BOOTSTAGE_ID_NET_ETH_START);
+#if CONFIG_IS_ENABLED(ETH)
 #if defined(CONFIG_MII) || defined(CONFIG_CMD_MII) || defined(CONFIG_PHYLIB)
 	miiphy_init();
 #endif
@@ -39,6 +40,7 @@
 #ifdef CONFIG_PHYLIB
 	phy_init();
 #endif
+#endif
 }
 
 int eth_mac_skip(int index)