It is reasonable that elements may need to include additional
kernel modules in a dracut ramdisk. This is done with the
--add-drivers option to dracut, but previously the value passed
was hard-coded.
This change allows an element to put a file containing its desired
drivers in a dracut-drivers.d directory, and the list there will
be added to the list of drivers added. This functions in
essentially the same way as the binary-deps.d directory that
already exists for including additional executables in a ramdisk.
Change-Id: Ie892b908d36c175a469f7cde7dd803ad4b1942b6
