Kernel: Add 'boot_prof' option to enable full system profiling on boot

The full system profiling functionality is useful for profiling the
boot performance of the system. Add a new kernel boot option to start
the system with profiling enabled. This lets you disable and view a
profile once the system is booted.

You can use it by running:
```
$ run.sh qcmd boot_prof
```

Kernel/CommandLine.cpp

@@ -88,6 +88,11 @@ bool CommandLine::contains(const String& key) const
     return m_params.contains(key);
 }
 
+UNMAP_AFTER_INIT bool CommandLine::is_boot_profiling_enabled() const
+{
+    return contains("boot_prof");
+}
+
 UNMAP_AFTER_INIT bool CommandLine::is_ide_enabled() const
 {
     return !contains("disable_ide");