Move about 40 test files for FIT images (signing, external data,
split-elf, encryption, alignment, firmware loadables, templates) into
a fit/ subdirectory. Drop the numeric prefixes and the redundant fit_
filename prefix, and update all references.
Rename the three signature.dts variants to have unique names:
signature.dts, signature_multi_key.dts and signature_no_nodes.dts.
Signed-off-by: Simon Glass <simon.glass@canonical.com>
