sunxi-common.h: Don't undef CONFIG_CMD_NET only to redefine it again later

sunxi-common.h: Don't undef CONFIG_CMD_NET only to redefine it again later

config_distro_defaults.h which is include later will redefine CONFIG_CMD_NET,
drop the useless / meaningless undef of it.

While at also move the undef of CONFIG_CMD_FPGA up to directly under the
include of config_cmd_defaults.h, to make it clear that it overwrites
the setting done from config_cmd_defaults.h .

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>