Add bc for kernel build and comments

Dockerfile

@@ -5,20 +5,23 @@ ENV DEBIAN_FRONTEND=noninteractive TZ=Europe/Amsterdam
 RUN dpkg --add-architecture i386
 
 RUN apt-get update && \
-    apt-get -y install dh-autoreconf zip wget git build-essential make \
+    apt-get -y install dh-autoreconf zip wget git build-essential make bc \
     libc6:i386 libncurses5:i386 libstdc++6:i386 
 
 ENV ROOTDIR=/pocketbook-jailbreak
 WORKDIR ${ROOTDIR}
 
+# musl toolchain for static linking
 RUN wget https://toolchains.bootlin.com/downloads/releases/toolchains/armv7-eabihf/tarballs/armv7-eabihf--musl--stable-2018.11-1.tar.bz2
 RUN tar -xvf armv7-eabihf--musl--stable-2018.11-1.tar.bz2
 RUN rm armv7-eabihf--musl--stable-2018.11-1.tar.bz2
 
+# Pocketbook SDK toolchain
 RUN wget https://github.com/ezdiy/pocketbook-sdk5/archive/master.tar.gz
 RUN tar -xvf master.tar.gz
 RUN rm master.tar.gz
 
+# toolchain for kernel modules
 RUN wget https://releases.linaro.org/archive/15.05/components/toolchain/binaries/arm-linux-gnueabihf/gcc-linaro-4.9-2015.05-x86_64_arm-linux-gnueabihf.tar.xz
 RUN tar -xvf gcc-linaro-4.9-2015.05-x86_64_arm-linux-gnueabihf.tar.xz
 RUN rm gcc-linaro-4.9-2015.05-x86_64_arm-linux-gnueabihf.tar.xz