[samsung-maguro] Add firmware-samsung-maguro package

This PR adds the firmware-samsung-maguro package and remove the alpine linux-firmware package.

firmware-samsung-maguro installed size: 240 KiB linux-firmware installed size: 202.14 MiB

It also changes the location of the installed firmware files, now it uses the /lib/firmware/postmarketos/ folder.

I tested it on the device and wifi works. Also updated the device wiki page and added the steps to connect to a wifi network.