spl: introduce CONFIG_SPL_TARGET

Currently the SPL target is specified in a CPU-specific makefile
fragment.  While some targets may need something more complicated than a
simple target name, targets which don't need this shouldn't have to provide a makefile fragment just for this.

Signed-off-by: Scott Wood <scottwood@freescale.com>
---
v2: Removed default target as it's been pointed out to me how existing platforms
cause the SPL to be built.

README

@@ -2746,6 +2746,11 @@ FIT uImage format:
 		CONFIG_SPL_LIBGENERIC_SUPPORT
 		Support for lib/libgeneric.o in SPL binary
 
+		CONFIG_SPL_TARGET
+		Final target image containing SPL and payload.  Some SPLs
+		use an arch-specific makefile fragment instead, for
+		example if more than one image needs to be produced.
+
 Modem Support:
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