Allow CROSS to be set in environment in test subdirs, too.

[Paul Brook]

> >  # Most people will set this stuff on the command line, i.e.
> >  #        make CROSS=mipsel-linux-
> >  # will build uClibc for 'mipsel'.
> > +#
> > +# You can set CROSS in the environment, too.
> >
> > +ifndef CROSS
> >  CROSS      = $(subst ",, $(strip $(CROSS_COMPILER_PREFIX)))
> > +endif
>
> Command line variables already override ones in the makefile...

Yes, but *environment* variables do not.

Paul