Makefile.spl: Move generated AT91SAM NAND image boot.bin to spl directory

Makefile.spl: Move generated AT91SAM NAND image boot.bin to spl directory

This patch moves the AT91SAM NAND booting SPL image "boot.bin" which
includes the ECC values from the root directory into the spl directory,
where all SPL related images are located.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Andreas Bießmann <andreas@biessmann.org>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Tested on the taurus board:
Tested-by: Heiko Schocher <hs@denx.de>