net: Move CONFIG_E1000 options to Kconfig

Move config for the E1000 Ethernet driver to Kconfig and tidy up affected
boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/P5040DS_defconfig b/configs/P5040DS_defconfig
index 6df742e..5da5a3c 100644
--- a/configs/P5040DS_defconfig
+++ b/configs/P5040DS_defconfig
@@ -2,3 +2,5 @@
 CONFIG_MPC85xx=y
 CONFIG_TARGET_P5040DS=y
 CONFIG_SPI_FLASH=y
+CONFIG_NETDEVICES=y
+CONFIG_E1000=y