libbladeRF: [WIP] Moved most remaining logic to bladerf_priv.c

bladerf.c now contains mostly device lock acquisition and calls
down into other modules. Core code should no longer call up to
any of these API functions that take a device handle as an argument.

bladerf_priv.c provides the glue logic between the various sub-modules.

Flash field-related routines have been moved to flash_fields.c.