firmware_loader: enhance Kconfig documentation over FW_LOADER

firmware_loader: enhance Kconfig documentation over FW_LOADER

If you try to read FW_LOADER today it speaks of old riddles and
unless you have been following development closely you will lose
track of what is what. Even the documentation for PREVENT_FIRMWARE_BUILD
is a bit fuzzy and how it fits into this big picture.

Give the FW_LOADER kconfig documentation some love with more up to
date developments and recommendations. While at it, wrap the FW_LOADER
code into its own menu to compartmentalize and make it clearer which
components really are part of the FW_LOADER. This should also make
it easier to later move these kconfig entries into the firmware_loader/
directory later.

This also now recommends using firmwared [0] for folks left needing a
uevent handler in userspace for the sysfs firmware fallback mechanis
given udev's uevent firmware mechanism was ripped out a while ago.

[0] https://github.com/teg/firmwared

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>