[U-Boot] Add _end symbol in SMDK6410 link script

[seedshope]

Since we rename _end to __bss_end (commit: 44c6e6591cb)
So we have add the the end symbol of u-boot image.

The patch fix SMDK6400 build error. the error information as foolowing:

pat.o -L /usr/local/arm/4.3.1-eabi-armv6/usr/bin-ccache/../lib/gcc/arm-samsung-linux-gnueabi/4.3.1 -lgcc -Map u-boot.map -o u-boot
arch/arm/cpu/arm1176/start.o: In function `_end_ofs':
/home/seedshope/work/bootloader/u-boot/arch/arm/cpu/arm1176/start.S:61: undefined reference to `_end'
make: *** [/home/seedshope/work/bootloader/u-boot_obj/smdk6400/u-boot] Error 1