board/aarch64: Build all network related drivers as modules

As we may need to load firmware to the PHYs, we can't load those
modules until we have access to the rootfs. In order for that to work,
the MDIO driver must not be built-in, as that will cause the kernel to
bind devices with missing (built-in) drivers to bind to the genphy
driver.

Therefore, build everything as modules and let udev load it during
boot.

board/aarch64/linux_defconfig

@@ -262,7 +262,7 @@ CONFIG_VETH=m
 CONFIG_VIRTIO_NET=y
 CONFIG_NLMON=y
 CONFIG_NET_VRF=y
-CONFIG_NET_DSA_MV88E6XXX=y
+CONFIG_NET_DSA_MV88E6XXX=m
 CONFIG_NET_DSA_MV88E6XXX_PTP=y
 # CONFIG_NET_VENDOR_3COM is not set
 # CONFIG_NET_VENDOR_ADAPTEC is not set
@@ -287,8 +287,8 @@ CONFIG_NET_DSA_MV88E6XXX_PTP=y
 # CONFIG_NET_VENDOR_GOOGLE is not set
 # CONFIG_NET_VENDOR_HISILICON is not set
 # CONFIG_NET_VENDOR_HUAWEI is not set
-CONFIG_MVNETA=y
-CONFIG_MVPP2=y
+CONFIG_MVNETA=m
+CONFIG_MVPP2=m
 # CONFIG_NET_VENDOR_MELLANOX is not set
 # CONFIG_NET_VENDOR_MICREL is not set
 CONFIG_SPARX5_SWITCH=y
@@ -326,11 +326,11 @@ CONFIG_8139CP=y
 # CONFIG_NET_VENDOR_XILINX is not set
 CONFIG_SFP=y
 CONFIG_MARVELL_PHY=y
-CONFIG_MARVELL_10G_PHY=y
+CONFIG_MARVELL_10G_PHY=m
 CONFIG_MICREL_PHY=y
 CONFIG_MICROCHIP_PHY=y
 CONFIG_MICROCHIP_T1_PHY=y
-CONFIG_MICROSEMI_PHY=y
+CONFIG_MICROSEMI_PHY=m
 CONFIG_MDIO_BITBANG=y
 CONFIG_MDIO_MVUSB=y
 CONFIG_MDIO_MSCC_MIIM=y