Install Arch Linux with wifi but wifi chipset not detected

Trying to install Arch Linux on a partition of my hard drive, my only option is to connect to Internet with WiFi. Ubuntu (on another partition) shows with lspci that WiFi chip-set is Broadcom BCM4311 and lspci -k shows Ubuntu is using ssb module and b43-pci-bridge driver which works fine. Arch Linux bootable USB with lspci -k shows that it is using the same module and driver but ifconfig shows no wireless interface. [Here](https://wiki.archlinux.org/index.php/broadcom_wireless#b43) it is mentioned that BCM4311 works with [b43-firmware-classic](https://aur.archlinux.org/packages/b43-firmware-classic/) from AUR. Now, how can I install b43-firmware-classic to use my wireless chip-set, before even installing Arch Linux itself. I ran git clone on Ubuntu and downloaded Arch Linux b43-firmware-classic from AUR on my Ubuntu root partition. However I'm not sure where to go from there.