arm: socfpga: allow configs without network support

Currently, socfpga_common.h does not allow configurations without
network support. This is because CONFIG_CMD_PXE is defined in this
file and distro mode has DHCP hard-coded as available.

Fix this by moving CONFIG_CMD_PXE and CONFIG_MENU to the defconfigs
and by making DHCP optional in BOOT_TARGET_DEVICES(func).

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
diff --git a/configs/socfpga_de0_nano_soc_defconfig b/configs/socfpga_de0_nano_soc_defconfig
index 59b2dcf..4bfdf01 100644
--- a/configs/socfpga_de0_nano_soc_defconfig
+++ b/configs/socfpga_de0_nano_soc_defconfig
@@ -30,6 +30,7 @@
 CONFIG_CMD_USB=y
 CONFIG_CMD_USB_MASS_STORAGE=y
 CONFIG_CMD_DHCP=y
+CONFIG_CMD_PXE=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_CACHE=y