[bootlin/training-materials] 119c3c: slides/sysdev-kernel-building: fix cleanup targets

[Michael Opdenacker]

  Branch: refs/heads/master
  Home:   https://github.com/bootlin/training-materials
  Commit: 119c3c606d59304aed5f6ce85c6f449e1eb4f15d
      https://github.com/bootlin/training-materials/commit/119c3c606d59304aed5f6ce85c6f449e1eb4f15d
  Author: Michael Opdenacker <michael.opdenacker at bootlin.com>
  Date:   2022-09-01 (Thu, 01 Sep 2022)

  Changed paths:
    M slides/sysdev-kernel-building/sysdev-kernel-building.tex

  Log Message:
  -----------
  slides/sysdev-kernel-building: fix cleanup targets

- "make mrproper" no longer necessary before switching architectures
- "make clean" was incomplete. Use the output of "make help"

Signed-off-by: Michael Opdenacker <michael.opdenacker at bootlin.com>