firmware loader: allow disabling of udev as firmware loader

[The patch was originally proposed by Tom Gundersen, and rewritten
 afterwards by me; most of changelogs below borrowed from Tom's
 original patch -- tiwai]

Currently (at least) the dell-rbu driver selects FW_LOADER_USER_HELPER,
which means that distros can't really stop loading firmware through
udev without breaking other users (though some have).

Ideally we would remove/disable the udev firmware helper in both the
kernel and in udev, but if we were to disable it in udev and not the
kernel, the result would be (seemingly) hung kernels as no one would
be around to cancel firmware requests.

This patch allows udev firmware loading to be disabled while still
allowing non-udev firmware loading, as done by the dell-rbu driver, to
continue working. This is achieved by only using the fallback
mechanism when the uevent is suppressed.

The patch renames the user-selectable Kconfig from FW_LOADER_USER_HELPER
to FW_LOADER_USER_HELPER_FALLBACK, and the former is reverse-selected
by the latter or the drivers that need userhelper like dell-rbu.

Also, the "default y" is removed together with this change, since it's
been deprecated in udev upstream, thus rather better to disable it
nowadays.

Tested with
    FW_LOADER_USER_HELPER=n
    LATTICE_ECP3_CONFIG=y
    DELL_RBU=y
and udev without the firmware loading support, but I don't have the
hardware to test the lattice/dell drivers, so additional testing would
be appreciated.

Reviewed-by: Tom Gundersen <teg@jklm.no>
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Abhay Salunke <Abhay_Salunke@dell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kay Sievers <kay@vrfy.org>
Tested-by: Balaji Singh <B_B_Singh@DELL.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>