image-host: refactor and protect for very long filenames

This patch adds a function fit_image_read_key_iv_data that checks the
return value of snprintf and allows to generate a sensible error
message when generating binary images using filenames that are too
long for the OS to handle.

This is especially relevant for automated builds such as Buildroot and
Yocto builds.

Signed-off-by: Hugo Cornelis <hugo.cornelis@essensium.com>