configs: set CONFIG_NET=y when PXE is enabled

Many platforms won't build when NET_LWIP is the default choice in
Kconfig:

 include/config_distro_bootcmd.h:443:9: error: expected '}' before 'BOOT_TARGET_DEVICES_references_PXE_without_CONFIG_CMD_DHCP_or_PXE'
 [...]

Set the legacy stack (CONFIG_NET=y) for these platforms.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>