usbmount: install configuration file unconditionally

As discussed, users should use a rootfs overlay or a post-build script
instead of a custom skeleton to override files installed by Buildroot,
so there is no point in having conditions when installing init scripts
or configuration files.

[Peter: fix indentation as noted by Yann]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>