Driver core: make uevent-environment available in uevent-file

This allows sysfs to show the environment variables that are available
if the uevent happens.  This lets userspace not have to cache all of
this information as the kernel already knows it.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>