Signature verification fails when BOOT_SIGNATURE_TYPE_PURE is used and NVM does not start at 0x0

If BOOT_SIGNATURE_TYPE_PURE option is enabled and the NVM memory does not start at 0x0, then bootutil_img_validate() fails. The problem is that we pass wrong memory address to bootutil_verify_img() - we do not take into account flash base address but only image flash offset.