package/linux-firmware: reorganise config menus

Reorganise the sub-menus for the linux-firmware package, so it is easier
to add new categories of firmwares to install (coming in a future patch).

Remove the per-chipco sub-menus, as all entries have the chipco name in
its prompt (except for TI, which is added).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>