Generally with Unix programs, "-h" or "--help" will give you syntax information, though it differs from program to program.
make is a program which, basically, runs a set of commands that are stipulated in the
Makefile. You can see that "veryclean" is a
target in the Makefile, which doesn't do anything itself, but depends upon the "clean" and "toolsclean" targets, causing them to be run.
veryclean: clean toolsclean
toolsclean:
$(SILENT)$(MAKE) -C $(TOOLSDIR) clean
clean:
$(SILENT)echo Cleaning build directory
$(SILENT)rm -rf rockbox.zip TAGS apps firmware comsim sim lang.[ch] manual *.pdf *.a credits.raw rockbox.ipod bitmaps pluginbitmaps UI256.bmp rockbox-full.zip html txt rockbox-manual*.zip sysfont.h rockbox-info.txt voicefontids *.wav *.mp3 *.voice max_language_size.h
Your Makefile will be different depending on what sort of build you are making, since it is generated by the rockbox/tools/configure script.