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/common/spl/Kconfig b/common/spl/Kconfig
index c43df88..34e8a12 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -585,6 +585,7 @@
 config SPL_ETH
 	bool "Support Ethernet"
 	depends on SPL_ENV_SUPPORT
+	depends on SPL_NET
 	help
 	  Enable access to the network subsystem and associated Ethernet
 	  drivers in SPL. This permits SPL to load U-Boot over an Ethernet
@@ -1216,6 +1217,7 @@
 
 config SPL_USB_ETHER
 	bool "Support USB Ethernet drivers"
+	depends on SPL_NET
 	help
 	  Enable access to the USB network subsystem and associated
 	  drivers in SPL. This permits SPL to load U-Boot over a