targets/*.conf cleanup: remove unnecessary TPUDRV_blob= lines MAIN_blob= and/or TPUDRV_blob= may be *optionally* defined in targets/*.conf files; if either variable is defined, it means that using the specified version of the blob library is allowed on this target, albeit likely suboptimal; if the variable is not defined, then using a blob is not an option and compilation from source is required for this component. MAIN_blob= and TPUDRV_blob= are now defaulted to empty in configure.sh, thus there is no need to explicitly specify that using a blob is not a valid option.