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/board/mousse/u-boot.lds.rom b/board/mousse/u-boot.lds.rom
index 4f42b32..b4b9e02 100644
--- a/board/mousse/u-boot.lds.rom
+++ b/board/mousse/u-boot.lds.rom
@@ -126,6 +126,6 @@
    *(COMMON)
   }
 
-  _end = . ;
+  __bss_end__ = . ;
   PROVIDE (end = .);
 }