arm: move exception handling out of start.S files

Exception handling is basically identical for all ARM targets.
Factorize it out of the various start.S files and into a
single vectors.S file, and adjust linker scripts accordingly.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
diff --git a/arch/arm/cpu/u-boot-spl.lds b/arch/arm/cpu/u-boot-spl.lds
index 3e88668..4beddf0 100644
--- a/arch/arm/cpu/u-boot-spl.lds
+++ b/arch/arm/cpu/u-boot-spl.lds
@@ -18,6 +18,7 @@
 	.text :
 	{
 		__image_copy_start = .;
+		*(.vectors)
 		CPUDIR/start.o (.text*)
 		*(.text*)
 	}