vexpress64: Fix bootargs when building without NET

vexpress64: Fix bootargs when building without NET

When building without DHCP/PXE configurations (NET disabled),
compilation errors may occur due to mismatched bootargs.
Ensure bootargs related to DHCP/PXE are not enabled if the
corresponding commands are disabled.

include/config_distro_bootcmd.h:443:9: error: expected ‘}’ before
‘BOOT_TARGET_DEVICES_references_PXE_without_CONFIG_CMD_DHCP_or_PXE’
  443 |         BOOT_TARGET_DEVICES_references_PXE_without_CONFIG_CMD_DHCP_or_PXE
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Chanho Park <parkch98@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>