scripts: imgtool: Fix img verify for hex file format

Currently imgtool --verify fails for hex files with:

Invalid image magic; is this an MCUboot image?

Added support for hex files by converting hex to bin
using IntelHex::tobinstr().

Reusing image.load() needs a bit of rework, maybe a
common load method will be done in the future,

Signed-off-by: Lucian Zala <zala.lucian@gmail.com>