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>