duma: fix downloading by forcing extension to .tar.gz

Downoading a non-existing file from sourceforge gives you a "200 OK"
and an index.html. As we try to retrieve a .tar.bz2 first, and duma
is bundled in a .tar.gz, we won't get appropriate content, so
just force the extension to avoid the problem.

Thanks to Ingmar Schraub <is@eseco.de> for pointing out the issue.

 # Kernel options

 menu "Operating System"

 # Config option used throughout the config and code to determine wether

 # we have a kernel or not (there might be different bare metal stuff)...

 config BARE_METAL

     bool

     default n

 config KERNEL

     string

 config KERNEL_VERSION

     string

 source config.gen/kernel.in

 endmenu