Im trying to make tufte-book to use EB Garamond. It does, but sadly, i have quite a few overfull boxes because many packages/styles use bold fonts.

An MWE for the complaint would be this:

```
\documentclass[a4paper,nobib,twoside,nofonts,nols]{tufte-book}

\usepackage{ebgaramond}

\ifxetex
  \usepackage{fontspec}
  \setmainfont{EB Garamond}
  % some more renewals, as suggested in http://tex.stackexchange.com/a/202189/9293
  % \renewcommand{\bfseries}[1]{\swshape}

\fi

\begin{document}
\part{Whine and cheese}
\end{document}

```

The above mwe compiles with the warnings:

```
LaTeX Font Warning: Font shape `EU1/EBGaramond(0)/bx/n' undefined
(Font)              using `EU1/EBGaramond(0)/m/n' instead on input line 32.

[1] [2] (index.aux)stdin -> index.pdf
[1][2]
4341 bytes written

LaTeX Font Warning: Some font shapes were not available, defaults substituted.

```

If i use my full document, its getting worse, because quite a few line breaks are at the wrong places (i suspect the font substitution to be the culprit).

I already tried to renew the `\bfseries` command, as i suspect it to be responsible to be used in the `book.sty`, but if i do, the text for the part changes (first few chars are omitted, and "Part" is dropped, too), probably because redefining `\bfseries` does more than setting font weight :/

Therefore i ask you: 
a) How to redefine the acutal font parameterization only for `\part`, and
b) How to redefine all bold for all usage of ebgaramond? (Because EB Garamond does not support `bx`)

or in general: How to fix such an problem?