sysreset: Add SYSRESET_TO_FIRMWARE_UI with reset -u support

Add new SYSRESET_TO_FIRMWARE_UI reset type to allow resetting directly
to firmware UI. This is implemented via the reset command's new -u flag.

For the EFI app, this sets the EFI_OS_INDICATIONS_BOOT_TO_FW_UI bit in
the OsIndications variable before performing a warm reset, causing the
firmware to boot to its setup interface.

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

cmd/boot.c

@@ -61,6 +61,7 @@ U_BOOT_CMD(
 	"Perform RESET of the CPU",
 	"- cold boot without level specifier\n"
 	"reset -h - hotreset if implemented\n"
+	"reset -u - reset to firmware UI if implemented\n"
 	"reset -w - warm reset if implemented"
 );