efi: Add a command to help with shim

Shim is a program which normally comes before U-Boot in the boot
process. But when the app runs first, it can sometimes chain Shim since
that is what is contained within the bootaa64.efi file, for example.

Add a simple command for dealing with shim. For now it only supports
enabling verbosity.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/shim.rst

@@ -0,0 +1,101 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+shim command
+============
+
+Synopsis
+--------
+
+::
+
+    shim debug [<0/1>]
+
+Description
+-----------
+
+The ``shim`` command provides utilities for controlling and configuring the
+UEFI Shim bootloader when U-Boot is running as an EFI application. Shim is a
+UEFI bootloader that is used to verify and boot signed operating system
+loaders, commonly used in secure boot environments.
+
+The ``shim debug`` subcommand manages the Shim verbose debugging mode through
+the ``SHIM_VERBOSE`` EFI variable.
+
+Subcommands
+~~~~~~~~~~~
+
+shim debug
+^^^^^^^^^^
+
+Controls the Shim verbose debugging mode.
+
+**Usage:**
+
+::
+
+    shim debug          # Display current debug state (0 or 1)
+    shim debug 0        # Disable verbose debugging
+    shim debug 1        # Enable verbose debugging
+
+The command reads from or writes to the ``SHIM_VERBOSE`` EFI variable in the
+Shim Lock GUID namespace. When verbose mode is enabled (value = 1), Shim will
+output additional debugging information during the boot process. When disabled
+(value = 0 or variable doesn't exist), Shim operates silently.
+
+**Parameters:**
+
+* ``<0/1>`` - Optional parameter to set debug mode:
+
+  * ``0`` - Disable verbose debugging
+  * ``1`` - Enable verbose debugging
+  * If omitted, displays current debug state
+
+**Return value:**
+
+The command returns 0 on success, 1 on failure.
+
+**Examples:**
+
+Check current debug state::
+
+    => shim debug
+    0
+
+Enable verbose debugging::
+
+    => shim debug 1
+
+Disable verbose debugging::
+
+    => shim debug 0
+
+Configuration
+~~~~~~~~~~~~~
+
+The shim command is available when:
+
+* ``CONFIG_CMD_SHIM`` is enabled
+* U-Boot is running as an EFI application
+* EFI-variable support is available
+
+Implementation Details
+~~~~~~~~~~~~~~~~~~~~~~
+
+The command uses the EFI variable services to read and write the
+``SHIM_VERBOSE`` variable with the following characteristics:
+
+* **Variable Name:** ``SHIM_VERBOSE`` (Unicode string)
+* **GUID:** EFI Shim Lock GUID (``605dab50-e046-4300-abb6-3dd810dd8b23``)
+* **Attributes:** ``EFI_VARIABLE_BOOTSERVICE_ACCESS``
+* **Data Type:** 32-bit unsigned integer (4 bytes)
+* **Values:** 0 (disabled) or 1 (enabled)
+
+The variable is stored in the EFI variable store and persists across reboots
+until explicitly changed or the variable store is cleared.
+
+See Also
+~~~~~~~~
+
+* :doc:`efidebug` - EFI debugging utilities
+* :doc:`bootefi` - Boot EFI applications
+* :doc:`/develop/uefi/u-boot_on_efi`