libbladeRF: Move sha256 into host/common
libbladeRF: Factor out file_* functions from image.c into file_ops.c
libbladeRF: Move variable delarations to beginning of scope

..because fo fucking MSVC
libbladeRF: Fix unchecked malloc return values
libbladeRF: Fix some assertions
libbladeRF: Add device serial to flash image on backup
libbladeRF: Fix some typos
libbladeRF: int32_t -> bladerf_image_meta_tag where appropriate
libbladeRF: Add docstring for BLADERF_SIGNATURE_SIZE