Move SeaBIOS post/boot stack to avoid conflict with gPXE.

It appears gPXE also uses a stack at 0x7c00.  gPXE probably shouldn't
    do that, but for now, move SeaBIOS stack to 0x7000.
diff --git a/src/config.h b/src/config.h
index 3033133..05f3942 100644
--- a/src/config.h
+++ b/src/config.h
@@ -128,7 +128,7 @@
 #define CONFIG_BIOS_REVISION 0x01
 
 // Various memory addresses used by the code.
-#define BUILD_STACK_ADDR          0x7c00
+#define BUILD_STACK_ADDR          0x7000
 #define BUILD_S3RESUME_STACK_ADDR 0x1000
 #define BUILD_AP_BOOT_ADDR        0x10000
 #define BUILD_EBDA_MINIMUM        0x90000