[PATCH] v3: factor out common stage0 code

We have lots of bit-for-bit identical code in the various stage0
implementations. Factor out the 16 bit code with associated protected
mode activation.

I'm open to moving even more common code out, but this is a well-defined
start.
If you think the stage0_common.S name is bad/evil/dumb, you're welcome
to suggest an alternative. No problem from my side.

This cleanup has been prepared for by r902, r905 and r907.

Boot tested on qemu. Build tested on i586, geodelx, k8.

The diffstat is most enlightening:
 amd/stage0.S     |  145 ---------------------------------------------
 geodelx/stage0.S |  145 ---------------------------------------------
 i586/stage0.S    |  145 ---------------------------------------------
 stage0_common.S  |  145 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 148 insertions(+), 432 deletions(-)

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006 <at> gmx.net>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006 <at> gmx.net>

Index: corebootv3-arch_x86_cleanup/arch/x86/geodelx/stage0.S
===================================================================
--- corebootv3-arch_x86_cleanup/arch/x86/geodelx/stage0.S	(Revision 907)
+++ corebootv3-arch_x86_cleanup/arch/x86/geodelx/stage0.S	(Arbeitskopie)
@@ -28,112 +28,7 @@
 #include <macros.h>
 #include <amd_geodelx.h>