barebox: support multi-image-build image selection

barebox: support multi-image-build image selection

Support optional selection of the built image filename in a
multi-image barebox build. This makes it possible to specify which
image to pick in a multi-image barebox config such as the
am335x_defconfig.

Signed-off-by: Pieter Smith <pieter@boesman.nl>
[Thomas:
 - remove default "", since this is the default for string options
 - rename the prompt from "Barebox image filename" to "Image filename"
 - slightly improve the Config.in help text.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>