scripts/mkimg.standard.sh: add linux-firmware-none to the extended iso
Fixes installation without network, if this pkg is not on the iso:
Installing system on /dev/xvda1:
extlinux: Not a directory: /mnt/boot
ERROR: unable to select packages:
linux-firmware-none (no such package):
required by: world[linux-firmware-none]
Part of #13973 (closed)
Edited by Henrik Riomar