[bootlin/training-materials] 8e217f: yocto-recipe-basics: mention S and B too

[Luca Ceresoli]

  Branch: refs/heads/master
  Home:   https://github.com/bootlin/training-materials
  Commit: 8e217f2e54e948121d9d31bdb5a3fed60f528bb9
      https://github.com/bootlin/training-materials/commit/8e217f2e54e948121d9d31bdb5a3fed60f528bb9
  Author: Luca Ceresoli <luca at lucaceresoli.net>
  Date:   2022-03-11 (Fri, 11 Mar 2022)

  Changed paths:
    M slides/yocto-recipe-basics/yocto-recipe-basics.tex

  Log Message:
  -----------
  yocto-recipe-basics: mention S and B too

S and B are important variables, e.g. whenever a package needs a custom
do_compile(), so mention them.

Also move WORKDIR at the top as it logically is the "entry point" to the
other directories.

Signed-off-by: Luca Ceresoli <luca at lucaceresoli.net>