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/cpu/ppc4xx/start.S b/arch/powerpc/cpu/ppc4xx/start.S
index 2218508..b43e22c 100644
--- a/arch/powerpc/cpu/ppc4xx/start.S
+++ b/arch/powerpc/cpu/ppc4xx/start.S
@@ -248,7 +248,7 @@
 	GOT_ENTRY(transfer_to_handler)
 
 	GOT_ENTRY(__init_end)
-	GOT_ENTRY(_end)
+	GOT_ENTRY(__bss_end__)
 	GOT_ENTRY(__bss_start)
 	END_GOT
 #endif /* CONFIG_NAND_SPL */
@@ -1624,7 +1624,7 @@
 	 * Now clear BSS segment
 	 */
 	lwz	r3,GOT(__bss_start)
-	lwz	r4,GOT(_end)
+	lwz	r4,GOT(__bss_end__)
 
 	cmplw	0, r3, r4
 	beq	7f