arm: at91: siemens: Add support to generate combined SPL+U-Boot image

arm: at91: siemens: Add support to generate combined SPL+U-Boot image

This patch adds the necessary defines to the Siemens AT91SAM based
boards (smartweb, corvus and taurus) to generate the combined binary
image with SPL and main U-Boot image combined (u-boot-with-spl.bin).

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>