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>