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/nand_spl/board/sheldon/simpc8313/u-boot.lds b/nand_spl/board/sheldon/simpc8313/u-boot.lds
index 1da4287..534a0c8 100644
--- a/nand_spl/board/sheldon/simpc8313/u-boot.lds
+++ b/nand_spl/board/sheldon/simpc8313/u-boot.lds
@@ -48,7 +48,7 @@
 	. = ALIGN(8);
 	__bss_start = .;
 	.bss (NOLOAD) : { *(.*bss) }
-	_end = .;
+	__bss_end__ = .;
 }
 ENTRY(_start)
-ASSERT(_end <= 0xfff01000, "NAND bootstrap too big");
+ASSERT(__bss_end__ <= 0xfff01000, "NAND bootstrap too big");