arc: fs-overlay: Force DirectFB to use legacy FBdev

DirectFB now (since
DirectFB/directfb@7fd0c2d)
used "/dev/dri/card0" by default if there's a DRM GPU driver
instantiated in the system.

But drmkms system driver doesn’t know physical framebuffer address
and so DirectFB further during execution will fail like that:
-------------------------->8----------------------
(!) System/DevMem: Please supply 'video-phys = 0xXXXXXXXX' and
'video-length = XXXX' options!
(!) DirectFB/Core: Could not initialize 'system_core' core!
    --> An invalid argument has been specified
(!!!)  *** UNIMPLEMENTED [fusion_dispatch] *** [fusion.c:3937]
df_dok.c <1749>:
        (#) DirectFBError [DirectFBCreate( &dfb )]: An invalid argument
has been specified
-------------------------->8----------------------

We work-around that by forcing DirectFB to use /dev/fbdev0.

Signed-off-by: Alexey Brodkin <[email protected]>

Author: abrodkin

board/synopsys/axs10x/fs-overlay/etc/directfbrc

@@ -0,0 +1 @@
+system=fbdev