Currently mkbootdisk can not make boot image as a file.
It would be nice if 

 mkbootdisk --device file.img 2.4.18-5

would save image to file.
(in a similar way as syslinux does)

The required changes in mkbootdisk are very simple.

For example this "quick hack" allows saving a boot image to a file.

1. take any existing image, like boot.img and copy it to file.img
2. edit mkbootdisk 
    and replace in /sbin/mkbootdisk
mount -t vfat $device $MOUNTDIR
by
mount -o loop -t vfat $device $MOUNTDIR

After this "quick hack" (which only demostrates the
simplicity of the new functionality implementation)
mkbootdisk writes a boot image to file.img

So all the infrastructure is already there,
only few little changes in option parsing 
are required to make it working in a nice clean way.