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>

arch/arm/lib/crt0_aarch64_efi.S

@@ -66,7 +66,11 @@ extra_header_fields:
 	.long	_start - ImageBase		/* SizeOfHeaders */
 	.long	0				/* CheckSum */
 	.short	IMAGE_SUBSYSTEM_EFI_APPLICATION /* Subsystem */
+#if CONFIG_VENDOR_EFI
 	.short	0				/* DllCharacteristics */
+#else
+	.short	IMAGE_DLLCHARACTERISTICS_NX_COMPAT
+#endif
 	.quad	0				/* SizeOfStackReserve */
 	.quad	0				/* SizeOfStackCommit */
 	.quad	0				/* SizeOfHeapReserve */

arch/arm/lib/crt0_arm_efi.S

@@ -65,7 +65,11 @@ extra_header_fields:
 	.long	_start - image_base		/* SizeOfHeaders */
 	.long	0				/* CheckSum */
 	.short	IMAGE_SUBSYSTEM_EFI_APPLICATION	/* Subsystem */
+#if CONFIG_VENDOR_EFI
 	.short	0				/* DllCharacteristics */
+#else
+	.short	IMAGE_DLLCHARACTERISTICS_NX_COMPAT
+#endif
 	.long	0				/* SizeOfStackReserve */
 	.long	0				/* SizeOfStackCommit */
 	.long	0				/* SizeOfHeapReserve */

arch/riscv/lib/crt0_riscv_efi.S

@@ -96,7 +96,11 @@ extra_header_fields:
 	.long	_start - ImageBase		/* SizeOfHeaders */
 	.long	0				/* CheckSum */
 	.short	IMAGE_SUBSYSTEM_EFI_APPLICATION /* Subsystem */
+#if CONFIG_VENDOR_EFI
 	.short	0				/* DllCharacteristics */
+#else
+	.short	IMAGE_DLLCHARACTERISTICS_NX_COMPAT
+#endif
 #if __riscv_xlen == 32
 	.long	0				/* SizeOfStackReserve */
 	.long	0				/* SizeOfStackCommit */

include/asm-generic/pe.h

@@ -51,6 +51,9 @@
 #define IMAGE_SUBSYSTEM_EFI_RUNTIME_DRIVER	12
 #define IMAGE_SUBSYSTEM_EFI_ROM			13
 
+/* DLL characteristics */
+#define IMAGE_DLLCHARACTERISTICS_NX_COMPAT	0x100
+
 /* Section flags */
 #define IMAGE_SCN_CNT_CODE			0x00000020
 #define IMAGE_SCN_CNT_INITIALIZED_DATA		0x00000040