HelloUefi: Add 30 second stall (#120)

## Description

This application is the simplest UEFI application possible, with no
dependencies. These changes add a 30 second stall such that a platform
may boot directly to this application and see "Hello UEFI!" with no need
for a shell to run it (Such as ShellPkg). Futher This

- [ ] Impacts functionality?
- [ ] Impacts security?
- [ ] Breaking change?
- [ ] Includes tests?
- [X] Includes documentation?
  - readme.md

## How This Was Tested

Ran on a physical machine, QemuQ35,

built With GCC and VS2022
 
## Integration Instructions

<_Describe how these changes should be integrated. Use N/A if nothing is
required._>

---------

Co-authored-by: Michael Kubacki <[email protected]>

OemPkg/HelloUefi/HelloUefi.c

@@ -1,6 +1,6 @@
 /** @file
   This sample application that is the simplest UEFI application possible.
-  It simply prints "Hello Uefi!" to the UEFI Console Out device.
+  It simply prints "Hello Uefi!" to the UEFI Console Out device and stalls the CPU for 30 seconds.
 
   Copyright (C) Microsoft Corporation
   SPDX-License-Identifier: BSD-2-Clause-Patent
@@ -9,6 +9,9 @@
 #include <Uefi.h>
 #include <Library/UefiApplicationEntryPoint.h>
 
+// 30 seconds in microseconds
+#define STALL_30_SECONDS 30000000
+
 /**
   The user Entry Point for Application. The user code starts with this function
   as the real entry point for the application.
@@ -30,14 +33,30 @@ UefiMain (
 {
   EFI_STATUS  Status;
 
-  if ((SystemTable == NULL) || (SystemTable->ConOut == NULL) || (SystemTable->ConOut->OutputString == NULL)) {
+  if (SystemTable == NULL) {
+
     return EFI_INVALID_PARAMETER;
   }
 
-  Status = SystemTable->ConOut->OutputString (SystemTable->ConOut, L"Hello Uefi!\r\n");
+  if ((SystemTable->ConOut == NULL) || (SystemTable->ConOut->OutputString == NULL) || (SystemTable->ConOut->ClearScreen == NULL)) {
+    return EFI_INVALID_PARAMETER;
+  }
+
+  if ((SystemTable->BootServices == NULL) || (SystemTable->BootServices->Stall == NULL)) {
+    return EFI_INVALID_PARAMETER;
+  }
+
+  Status = SystemTable->ConOut->ClearScreen (SystemTable->ConOut);
+  if (EFI_ERROR (Status)) {
+    return Status;
+  }
+
+  Status = SystemTable->ConOut->OutputString (SystemTable->ConOut, L"\r\nHello Uefi!\r\n");
   if (EFI_ERROR (Status)) {
     return Status;
   }
 
+  SystemTable->BootServices->Stall (STALL_30_SECONDS);
+
   return EFI_SUCCESS;
 }

OemPkg/HelloUefi/HelloUefi.inf

@@ -2,7 +2,7 @@
 #  Sample UEFI Application Reference Module.
 #
 # This sample application that is the simplest UEFI application possible.
-#  It simply prints "Hello Uefi!" to the UEFI Console Out device.
+# It simply prints "Hello Uefi!" to the UEFI Console Out device and stalls the CPU for 30 seconds.
 #
 #  Copyright (C) Microsoft Corporation
 #  SPDX-License-Identifier: BSD-2-Clause-Patent

OemPkg/HelloUefi/Readme.md

@@ -0,0 +1,19 @@
+# Simple Bootable Media Example
+
+Barebones example of how to boot to an EFI Application with no dependencies.
+
+## Boot Directions
+
+1. Format a usb drive as FAT32
+2. At the root of this usb drive create the following folders: `EFI/Boot/`
+3. Build and rename `HelloUefi.efi` as `boot<arch>.efi` and place at `EFI/BOOT/`
+
+    * IA32    - `bootx86.efi`
+    * AMD64   - `bootx64.efi`
+    * AARCH64 - `bootaa64.efi`
+
+4. On your platform, boot into UEFI Menu and change the boot order to boot USB first
+5. Turn off your platform
+6. Plug in usb drive
+7. Reboot
+8. If successful, you should now see `Hello Uefi!` in the top left corner