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_spl.lds b/arch/powerpc/cpu/mpc85xx/u-boot-nand_spl.lds
index b10e0f9..8410bd7 100644
--- a/arch/powerpc/cpu/mpc85xx/u-boot-nand_spl.lds
+++ b/arch/powerpc/cpu/mpc85xx/u-boot-nand_spl.lds
@@ -62,6 +62,6 @@
 		*(.sbss*)
 		*(.bss*)
 	}
-	_end = .;
+	__bss_end__ = .;
 }
 ASSERT(__init_end <= 0xfff00ffc, "NAND bootstrap too big");