kernel: Add bison/flex to kconfig Dockerfile
These are required for the 4.16.x kernel configuration.
FROM linuxkit/alpine:96ad1eb5ec262b4cd0eef574cdc0b225b502d9ee AS kernel-build
RUN apk add \
argp-standalone \
+ bison \
build-base \
curl \
diffutils \
+ flex \
libarchive-tools \
ncurses-dev \
patch \
