pkg/firmware: Use kernel v5.4.x as the base
this determines which firmware packages are included.
# Make modules from a recentish kernel available
-FROM linuxkit/kernel:4.14.126 AS kernel
+FROM linuxkit/kernel:5.4.28 AS kernel
FROM linuxkit/alpine:bc528cf9d4065d2e09aa44ff76909b94cfe8d867 AS build
RUN apk add --no-cache git kmod
