boot: image-pre-load: Check environment for location of signature info

Setting an alternative signature info node in "pre_load_sig_info_path"
allows verification of an image using the bootm pre-load mechanism with
a different key, e.g.: setenv pre_load_sig_info_path "/alt/sig" ; bootm
preload [addr]

Signed-off-by: Steven Lawrance <steven.lawrance@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>