classes: image_types_fsl: be more verbose in README
classes: image_types_fsl: be more verbose in README

The device tree handling on the sd-card can be confusing. Explain in
more detail how to change the default device tree while booting.

Signed-off-by: Stefan Christ <c04781dbce194802ec6a6311a1200fbcd0141e96@phytec.de>
Signed-off-by: Stefan Müller-Klieser <a3e08e35e4a42be768e7d0e76ae1ba321c2a66dd@phytec.de>

PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec