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/examples/standalone/mips.lds b/examples/standalone/mips.lds
index 63a1c92..68ae217 100644
--- a/examples/standalone/mips.lds
+++ b/examples/standalone/mips.lds
@@ -55,5 +55,5 @@
 	.sbss (NOLOAD) : { *(.sbss) }
 	.bss (NOLOAD)  : { *(.bss) . = ALIGN(4); }
 
-	_end = .;
+	__bss_end__ = .;
 }