yoctobuilder improvements and optimizations

• issue #55 (closed): added common_configs parameter to put all config parameters that apply to all builds (in the yaml example cache mirrors (readonly) from OSTC and local cache dirs are set)
• issue #56 (closed): added a logfile param to bash_live, so that the output is piped both to stdout and to a logfile
• issue #57 (closed): added rm_work as a common task for all recipes in all builds, in order to save space in "real time" and optimize pipeline disk usage
• issue #58 (closed): added flavour_dir and oe_init_build_env_dir parameters, for retro-compatibility with older versions of the project, and for compatibility with possible future changes
• updated image names (allscenarios-image-* -> oniro-image-*)