[SBS] Refactor wrapper script 'sbs'

- All sh functions are refactored.

- Boolean sh variables are used for all checks.

- Files '.sbs_debug' and '.sbs_verbose' are created in the build folder to remember used options, allowing to reconfigure only when necessary.

- Function 'clean' uses CMake only when appropriate and prints a message while removing build folder.

- Function 'flash' prints an error when the specified target is not flashable.