Kernel/aarch64: Support reading the command line via the RPi Mailbox

This reuses the existing `RPi::Mailbox` interface to read the command
line via a VideoCore-specific mailbox message. This will have to be
replaced if that interface starts being smarter, as this is needed very
early, and nothing guarantees that a smarter Mailbox interface wouldn't
need to allocate or log, which is a no-no during early boot.

As the response string can be arbitrarily long, it's the caller's job to
provide a long enough buffer for `Mailbox::query_kernel_command_line`.
This commit chose 512 bytes, as it provides a large enough headroom over
the 150-200 characters implicitly added by the VC firmware.

The portable way would be to parse the `/chosen/bootargs` property of
the device tree, but we currently lack the scaffolding for doing that.

Support for this in QEMU relies on a patch that has not yet been
accepted upstream, but is available via our `Toolchain/BuildQEMU.sh`
script. It should, however, work on bare metal.

Tested-By: Timon Kruiper <timonkruiper@gmail.com>

Kernel/Arch/aarch64/RPi/Mailbox.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/StringView.h>
 #include <AK/Types.h>
 
 namespace Kernel::RPi {
@@ -51,6 +52,9 @@ public:
     bool send_queue(void* queue, u32 queue_size) const;
 
     u32 query_firmware_version();
+
+    // Returns the kernel command line as a StringView into the given buffer.
+    StringView query_kernel_command_line(Bytes buffer);
 };
 
 }