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>
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v2: Removed default target as it's been pointed out to me how existing platforms
cause the SPL to be built.