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/arch/powerpc/lib/board.c b/arch/powerpc/lib/board.c
index b88cf6b..4719f8c 100644
--- a/arch/powerpc/lib/board.c
+++ b/arch/powerpc/lib/board.c
@@ -123,7 +123,7 @@
 #endif
 
 extern ulong __init_end;
-extern ulong _end;
+extern ulong __bss_end__;
 ulong monitor_flash_len;
 
 #if defined(CONFIG_CMD_BEDBUG)
@@ -403,7 +403,7 @@
 	 *  - monitor code
 	 *  - board info struct
 	 */
-	len = (ulong)&_end - CONFIG_SYS_MONITOR_BASE;
+	len = (ulong)&__bss_end__ - CONFIG_SYS_MONITOR_BASE;
 
 	/*
 	 * Subtract specified amount of memory to hide so that it won't