package/linux-firmware: add missing nvs file for the wl18xx firmware

The wl18xx shares the same wlcore as the wl1271 and needs the
wl1271-nvs.bin file.  Also, refactor some comments and list items.

Signed-off-by: Marc-Andre Chenier <mac@vrvana.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>