dfu: Make DFU support more SPL friendly

dfu: Make DFU support more SPL friendly

Do this by using $(SPL_) in Makefiles and CONFIG_IS_ENABLED in C code.
This ensures the files and features are only built into the right build
for which they are enabled. Using the macros to simplify this patch was
made possible by the config symbol rename done in the last patch.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Lukasz Majewski <lukma@denx.de>