Add _end for the end of u-boot image for SMDK6400

Since we rename _end to __bss_end__, But we need add _end symbol for
the end of u-boot image.
Signed-off-by: NZhong Hongbo <bocui107@gmail.com>
Signed-off-by: NMinkyu Kang <mk7.kang@samsung.com>

nand_spl/board/samsung/smdk6400/u-boot.lds

@@ -67,6 +67,8 @@ SECTIONS
 	*(.dynsym)
 	}
 
+	_end = .;
+
 	.bss __rel_dyn_start (OVERLAY) : {
 	__bss_start = .;
 	*(.bss)