Clean task removes also startup.o

[jukeks]

After executing Clean workspace task the environment is broken because all .o files under target were removed.

[prb28]

The clean workspace command, removes all the .o files from the build directory by design. Using a makefile as in this project you should declare a custom clean command in the makefile and avoid the built-in clean workspace command. This makefile command should be called in a custom vscode task.