[tlbuild] TL2010 banner

[Karl Berry]

  [...] TL_CONF_BANNER=--with-tex-banner='"TeX Live 2010/RC1 on Cygwin"' 

Try this:
TL_CONFIGURE_ARGS=--with-banner-add="/RC1 on Cygwin"

(I've now updated tug.org/texlive/build.html.)