Makefile: add rules to generate SPL FIT images

Some platforms require more complex U-Boot images than we can easily
generate via the mkimage command line, for instance to load additional
image files.
Introduce a CONFIG_SPL_FIT_SOURCE and CONFIG_SPL_FIT_GENERATOR symbol,
which can either hold an .its source file describing the image layout,
or, in the second case, a generator tool (script) to create such
a source file. This script gets passed the list of device tree files
from the CONFIG_OF_LIST variable.
A platform or board can define either of those in their defconfig file
to allow an easy building of such an image.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>