efi_loader: set IMAGE_DLLCHARACTERISTICS_NX_COMPAT

The IMAGE_DLLCHARACTERISTICS_NX_COMPAT flag marks an EFI binary where
the following conditions are met [1]:

* Executable and writable sections are separated.
* The application does not run self-modifying code.
* The application uses the EFI_MEMORY_ATTRIBUTE_PROTOCOL when loading
  executable code.
* The application does not assume that all memory ranges are usable.
* The stack is not expected to be executable.

The only EFI binaries U-Boot provides that do not fulfill these
requirements are the EFI app and the EFI payload.

Once we have implemented separation of writable and executable memory in
U-Boot we can use the IMAGE_DLLCHARACTERISTICS_NX_COMPAT flag to decide
if we will load an EFI binary.

[1] New UEFI CA memory mitigation requirements for signing
    https://techcommunity.microsoft.com/t5/hardware-dev-center/new-uefi-ca-memory-mitigation-requirements-for-signing/ba-p/3608714

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

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