x86/microcode: Document builtin microcode loading method

Add some text and an example to Documentation/x86/early-microcode.txt
explaining how to build in microcode.

Tested-by: Thomas Voegtle <tv@lio96.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1454499225-21544-18-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>