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/onenand_ipl/board/apollon/u-boot.onenand.lds b/onenand_ipl/board/apollon/u-boot.onenand.lds
index 0960c12..721d2f5 100644
--- a/onenand_ipl/board/apollon/u-boot.onenand.lds
+++ b/onenand_ipl/board/apollon/u-boot.onenand.lds
@@ -49,5 +49,5 @@
 	. = ALIGN(4);
 	__bss_start = .;
 	.bss : { *(.bss) . = ALIGN(4); }
-	_end = .;
+	__bss_end__ = .;
 }