[HALX86] Implement ONECPU boot switch (reactos#6024)

It causes the HAL to only use one (the boot) CPU on a MP system.
Attempting to start other processors will fail.

For more information, see:
https://www.geoffchappell.com/notes/windows/boot/bcd/osloader/numproc.htm
https://www.itprotoday.com/cloud-computing/what-switches-can-be-used-bootini
http://franck.kiechel.free.fr/dbr_eng/BootIni.htm
https://learn.microsoft.com/en-us/windows-hardware/drivers/devtest/bcdedit--set

hal/halx86/generic/halinit.c

@@ -14,6 +14,9 @@
 
 /* GLOBALS *******************************************************************/
 
+#ifdef CONFIG_SMP
+BOOLEAN HalpOnlyBootProcessor;
+#endif
 BOOLEAN HalpPciLockSettings;
 
 /* PRIVATE FUNCTIONS *********************************************************/
@@ -30,6 +33,12 @@ HalpGetParameters(
         /* Read the command line */
         PCSTR CommandLine = LoaderBlock->LoadOptions;
 
+#ifdef CONFIG_SMP
+        /* Check whether we should only start one CPU */
+        if (strstr(CommandLine, "ONECPU"))
+            HalpOnlyBootProcessor = TRUE;
+#endif
+
         /* Check if PCI is locked */
         if (strstr(CommandLine, "PCILOCK"))
             HalpPciLockSettings = TRUE;

hal/halx86/smp/i386/spinup.c

@@ -16,6 +16,8 @@
 
 /* GLOBALS *******************************************************************/
 
+extern BOOLEAN HalpOnlyBootProcessor;
+
 extern PPROCESSOR_IDENTITY HalpProcessorIdentity;
 extern PHYSICAL_ADDRESS HalpLowStubPhysicalAddress;
 extern PVOID HalpLowStub;
@@ -88,6 +90,11 @@ HalStartNextProcessor(
     _In_ PLOADER_PARAMETER_BLOCK LoaderBlock,
     _In_ PKPROCESSOR_STATE ProcessorState)
 {
+    /* Bail out if we only use the boot CPU */
+    if (HalpOnlyBootProcessor)
+        return FALSE;
+
+    /* Bail out if we have started all available CPUs */
     if (HalpStartedProcessorCount == HalpApicInfoTable.ProcessorCount)
         return FALSE;