arm, spl, at91: add spl support for the corvus board

replaces the at91bootstrap code with SPL code.
make the spl image with:
./tools/mkimage -T atmelimage -d spl/u-boot-spl.bin spl/boot.bin

this writes the length of the spl image into the 6th
execption vector. This is needed from the ROM bootloader.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Bo Shen <voice.shen@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ffc89e7..20d521a 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -203,6 +203,7 @@
 	select CPU_ARM926EJS
 
 config TARGET_CORVUS
+	select SUPPORT_SPL
 	bool "Support corvus"
 	select CPU_ARM926EJS