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/arm/cpu/arm720t/start.S b/arch/arm/cpu/arm720t/start.S
index c098118..c521753 100644
--- a/arch/arm/cpu/arm720t/start.S
+++ b/arch/arm/cpu/arm720t/start.S
@@ -91,7 +91,7 @@
 
 .globl _bss_end_ofs
 _bss_end_ofs:
-	.word _end - _start
+	.word __bss_end__ - _start
 
 #ifdef CONFIG_USE_IRQ
 /* IRQ stack memory (calculated at run-time) */