x86: Add an __end symbol to signal the end of the U-Boot binary

With this symbol we can easy append something (e.g. an FDT) to the U-Boot
binary and access it from within U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/include/asm/u-boot-x86.h b/arch/x86/include/asm/u-boot-x86.h
index 99062e5..948615d 100644
--- a/arch/x86/include/asm/u-boot-x86.h
+++ b/arch/x86/include/asm/u-boot-x86.h
@@ -31,6 +31,7 @@
 extern ulong __rel_dyn_end;
 extern ulong __bss_start;
 extern ulong __bss_end;
+extern ulong _end;
 
 /* cpu/.../cpu.c */
 int x86_cpu_init_r(void);