rename _end to __bss_end__

Currently, _end is used for end of BSS section.  We want _end to mean
end of u-boot image, so we rename _end to __bss_end__ first.

Signed-off-by: Po-Yu Chuang <ratbert@faraday-tech.com>
diff --git a/arch/powerpc/cpu/mpc85xx/u-boot-nand.lds b/arch/powerpc/cpu/mpc85xx/u-boot-nand.lds
index fa2088b..04bc731 100644
--- a/arch/powerpc/cpu/mpc85xx/u-boot-nand.lds
+++ b/arch/powerpc/cpu/mpc85xx/u-boot-nand.lds
@@ -101,6 +101,6 @@
   } :bss
 
   . = ALIGN(4);
-  _end = . ;
+  __bss_end__ = . ;
   PROVIDE (end = .);
 }