make firmware_install pollutes $(objtree) with files owned by root

[Nikanth Karthikesan <knikanth@suse.de>]

Hi

`make firmware_install` has to be run as root, so that the firmware could be 
installed in /lib/firmware. But it doesn't simply install firmware from the 
object directory. It first copies/converts files from source directory to the 
object directory, which then gets installed.

This leads to the situation where `make clean` cannot be run as normal user as 
he cannot remove files belonging to root. It would be a common expectaion to 
be able to delete the source/object tree as user.

We should introduce another make target, `make firmware` to convert/copy files 
from src dir to obj dir. Or this intermediate step should be avoided. Or 
atleast `make firmware_install` should delete the files it created in the 
object tree.

Thanks
Nikanth