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/eltec/mhpc/u-boot.lds b/board/eltec/mhpc/u-boot.lds
index 4a96388..2fac727 100644
--- a/board/eltec/mhpc/u-boot.lds
+++ b/board/eltec/mhpc/u-boot.lds
@@ -90,6 +90,6 @@
    *(COMMON)
    . = ALIGN(4);
   }
-  _end = . ;
+  __bss_end__ = . ;
   PROVIDE (end = .);
 }