exynos5420: add more firmware files and various fixes

A new firmware package called firmware-samsung-universal5420 is added, with firmware from https://github.com/exynos5420/android_vendor_samsung_universal5420-common. It contains common downstream firmware for all tablets based on exynos5420 (i.e. v1awifi, n1awifi, chagallwifi and so on).

This MR also adds wifi firmware for mainline kernel (nvram from downstream and blob from linux-firmware-brcm package).