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/netphone/u-boot.lds.debug b/board/netphone/u-boot.lds.debug
index df7b6ad..a001f3f 100644
--- a/board/netphone/u-boot.lds.debug
+++ b/board/netphone/u-boot.lds.debug
@@ -130,6 +130,6 @@
    *(.bss)
    *(COMMON)
   }
-  _end = . ;
+  __bss_end__ = . ;
   PROVIDE (end = .);
 }