common: image-sig.c: Remove host-specific logic and #ifdefs

Remove any ifdefs in image-sig.c that were previously used to
differentiate from the host code. Note that all code dedicated to
relocating ->sign() and ->add_verify_data)_ can be safely removed,
as signing is not supported target-side.

NOTE that although it appears we are removing ecdsa256 support, this
is intentional. ecdsa_verify() is a no-op on the target, and is
currently only used by host code.

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