firmware_loader: move struct builtin_fw to the only place used

Now that x86 doesn't abuse picking at internals to the firmware
loader move out the built-in firmware struct to its only user.

Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20211021155843.1969401-5-mcgrof@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

firmware_loader: formalize built-in firmware API

Formalize the built-in firmware with a proper API. This can later
be used by other callers where all they need is built-in firmware.

We export the firmware_request_builtin() call for now only
under the TEST_FIRMWARE symbol namespace as there are no
direct modular users for it. If they pop up they are free
to export it generally. Built-in code always gets access to
the callers and we'll demonstrate a hidden user which has been
lurking in the kernel for a while and the reason why using a
proper API was better long term.

Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20211021155843.1969401-2-mcgrof@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>