kernel-fitimage: mangle slashes to underscores as late as possible

This introduces no functional change, but will come in handy in a later
commit where a file lookup will have us using the device tree name. If
we keep it like it's now, we will lose the information whether an
underscore is an original underscore or a mangled slash.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>