Activating fi and fl ligatures with fontspec for Berling font

Question

I have a font named Berling containing fi and fl ligatures on glyph 109 and 110. How can I activate these with lualatex? For the moment I am trying:

\documentclass[a4paper]{memoir}
\usepackage[utf8]{inputenc}
\usepackage{fontspec}

\begin{document}
\setmainfont[ItalicFont={BerlIt}, Ligatures={Required}]{Berling}

Fi fi Fl fl \textit{Foo Fii fi}

\end{document}

But I get:

*************************************************
* fontspec warning: "icu-feature-not-exist-in-font"
* 
* OpenType feature 'Ligatures=Required' (+rlig) not available for font
* 'Berling' with script 'Latin' and language 'Default'.
*************************************************

---

EDIT: I have now also tried (and both with and without Script=Default)

\documentclass[a4paper]{memoir}
\usepackage{fontspec}

\begin{document}
\setmainfont[ItalicFont={BerlIt}, Ligatures={Common, TeX}, Script=Default]{Berling}

Fi fi Fl fl \textit{Foo Fii fi}

\end{document}

---

Answer 1

Thanks to egregs help and this answer the I got the following solution which works for me:

\documentclass[a4paper]{memoir}
\usepackage{fontspec}

\begin{document}
\setmainfont[ItalicFont={BerlIt}, FeatureFile=myligatures.fea]{Berling}

Fi fi Fl fl \textit{Foo Fii fi}

\end{document}

And the following feature file:

languagesystem DFLT dflt ;
languagesystem latn dflt ;
feature liga {
       sub f i by f_i ;
       sub f l by f_l ;
} liga ;

Now I am gonna take a last look at the glyphs in that file and see if there are any other ligatures worth using :)