common: Move host-only logic in image-sig.c to separate file

image-sig.c is used to map a hash or crypto algorithm name to a
handler of that algorithm. There is some similarity between the host
and target variants, with the differences worked out by #ifdefs. The
purpose of this change is to remove those ifdefs.

First, copy the file to a host-only version, and remove target
specific code. Although it looks like we are duplicating code,
subsequent patches will change the way target algorithms are searched.
Besides we are only duplicating three string to struct mapping
functions. This isn't something to fuss about.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>