ltq-vmmc: update permission handling

ltq-vmmc: update permission handling

The firmware is currently just copied. It can end up with o= on the
device (this is the case for voice_ar9_firmware.bin for instance).
Instead of copying it the Makefile is changed to use the macro
"$(INSTALL_DATA)" in order for the file to be world-readable.

While at it refactor the device node creation in the init script with
loop.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>
[removed 2nd part with custom group handling for device nodes]
Signed-off-by: Petr Štetiar <ynezz@true.cz>