build: Remove cmdline_dir and friends.

build: Remove cmdline_dir and friends.

This directory used to contain the source files of the command line
parsers generated by gengetopt. There are no such files any more,
so the the receipts which compile these source files can go away and
we do not need to create the directory any more.

The special CFLAGS for compiling those files can be removed as well
as the include directive for CPPFLAGS.

Finally, the make construct which filters out the .cmdline.d files
from the list of dependency files is no longer needed, and we may get
rid of the sed command which prepends $(cmdline_dir) to the includes
of the dependency files.