Add proposal for new security blueprint

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+# Consoles
+
+## Disable serial console
+
+Serial console output shall be disabled. To disable console output in U-Boot,
+set the following macros:
+
+<!-- config -->
+
+Domain                 | `Config` name                           | `Value`
+---------------------- | --------------------------------------- | ---------
+Boot-Consoles-Serial-1 | `CONFIG_SILENT_CONSOLE`                 | `Disable`
+Boot-Consoles-Serial-2 | `CONFIG_SYS_DEVICE_NULLDEV`             | `Disable`
+Boot-Consoles-Serial-3 | `CONFIG_SILENT_CONSOLE_UPDATE_ON_RELOC` | `Disable`
+
+<!-- endconfig --> <!-- todo -->
+
+Domain          | Improvement
+--------------- | ------------------------------------
+Boot-Consoles-1 | Secure loader: No reference earlier?
+
+<!-- endtodo -->
+
+And set "**silent**" environment variable. For the Secure loader, disable the
+traces by undefined the below macro:
+
+<!-- config -->
+
+Domain                 | `Environment variable` name | _State_
+---------------------- | --------------------------- | -------------
+Boot-Consoles-Serial-1 | `INC_DEBUG_PRINT`           | _Not defined_
+
+<!-- endconfig -->
+
+For sboot proper configuration needs to be done to disable the serial console.
+
+--------------------------------------------------------------------------------
+
+<!-- pagebreak -->
+
+## Immutable environment variables
+
+In U-Boot, ensure Kernel command line, boot commands, boot delay and other
+environment variables are immutable. This will prevent side-loading of alternate
+images, by restricting the boot selection to only the image in FLASH.
+
+The environment variables shall be part of the text region in U-Boot as default
+environment variable and not in non-volatile memory.
+
+Remove configuration options related to non-volatile memory, such as:
+
+<!-- config -->
+
+Domain                     | `Config` name                | _State_
+-------------------------- | ---------------------------- | ---------
+Boot-Consoles-Variables-1  | `CONFIG_ENV_IS_IN_MMC`       | `#undef`
+Boot-Consoles-Variables-2  | `CONFIG_ENV_IS_IN_EEPROM`    | `#undef`
+Boot-Consoles-Variables-3  | `CONFIG_ENV_IS_IN_FLASH`     | `#undef`
+Boot-Consoles-Variables-4  | `CONFIG_ENV_IS_IN_DATAFLASH` | `#undef`
+Boot-Consoles-Variables-5  | `CONFIG_ENV_IS_IN_FAT`       | `#undef`
+Boot-Consoles-Variables-6  | `CONFIG_ENV_IS_IN_NAND`      | `#undef`
+Boot-Consoles-Variables-7  | `CONFIG_ENV_IS_IN_NVRAM`     | `#undef`
+Boot-Consoles-Variables-8  | `CONFIG_ENV_IS_IN_ONENAND`   | `#undef`
+Boot-Consoles-Variables-9  | `CONFIG_ENV_IS_IN_SPI_FLASH` | `#undef`
+Boot-Consoles-Variables-10 | `CONFIG_ENV_IS_IN_REMOTE`    | `#undef`
+Boot-Consoles-Variables-11 | `CONFIG_ENV_IS_IN_UBI`       | `#undef`
+Boot-Consoles-Variables-12 | `CONFIG_ENV_IS_NOWHERE`      | `#define`
+
+<!-- endconfig -->
+
+--------------------------------------------------------------------------------
+
+<!-- pagebreak -->
+
+## (Recommendation) Removal of memory dump commands
+
+In U-Boot, following commands shall be disabled to avoid memory dumps:
+
+```
+md : Memory Display command.
+mm : Memory modify command - auto incrementing address.
+nm : Memory modify command - constant address.
+mw : Memory write.
+cp : Memory copy.
+mwc : Memory write cyclic.
+mdc : Memory display cyclic.
+mtest : Simple ram read/write test.
+loopw : Infinite write loop on address range.
+```
+
+<!-- config -->
+
+Domain                  | `Command` name | _State_
+----------------------- | -------------- | ----------
+Boot-Consoles-MemDump-1 | `md`           | _Disabled_
+Boot-Consoles-MemDump-2 | `mm`           | _Disabled_
+Boot-Consoles-MemDump-3 | `nm`           | _Disabled_
+Boot-Consoles-MemDump-4 | `mw`           | _Disabled_
+Boot-Consoles-MemDump-5 | `cp`           | _Disabled_
+Boot-Consoles-MemDump-6 | `mwc`          | _Disabled_
+Boot-Consoles-MemDump-7 | `mdc`          | _Disabled_
+Boot-Consoles-MemDump-8 | `mtest`        | _Disabled_
+Boot-Consoles-MemDump-9 | `loopw`        | _Disabled_
+
+<!-- endconfig -->
+
+Similarly, memory dump support shall be disabled from sboot.