src/update_handler: avoid fallback to img_to_raw_handler for special slot types
src/update_handler: avoid fallback to img_to_raw_handler for special slot types

Otherwise, the wrong handler might be run if support is disabled or a different
image type is supplied.

Also allow *.img for boot-mbr-switch.

Signed-off-by: Jan Luebbe <3a82f1ac4ad59607a20f180f1c67ac8ba31ad0d5@pengutronix.de>

jluebbe/rauc,ejoerns/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,jluebbe/rauc,rauc/rauc,rauc/rauc,ejoerns/rauc