Replace __bss_end__ with __bss_end

Note this is a tree-wide change affecting multiple architectures.

At present we use __bss_start, but mostly __bss_end__. This seems
inconsistent and in a number of places __bss_end is used instead.

Change to use __bss_end for the BSS end symbol throughout U-Boot. This
makes it possible to use the asm-generic/sections.h file on all
archs.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/powerpc/cpu/mpc86xx/u-boot.lds

@@ -90,6 +90,6 @@ SECTIONS
    *(COMMON)
    . = ALIGN(4);
   }
-  __bss_end__ = . ;
+  __bss_end = . ;
   PROVIDE (end = .);
 }