update dockerfile to reflect debian cross compilation changes

Dockerfile

@@ -2,20 +2,15 @@ FROM debian:jessie
 MAINTAINER Émile Morel
 
 # U-boot part
-RUN echo deb http://www.emdebian.org/debian unstable main > /etc/apt/sources.list.d/emdebian.list
 ENV DEBIAN_FRONTEND noninteractive
 ENV DEBCONF_NONINTERACTIVE_SEEN true
 ENV LC_ALL C
 ENV LANGUAGE C
 ENV LANG C
 RUN apt-get update
-RUN apt-get install --force-yes -y emdebian-archive-keyring
+RUN apt-get install --force-yes -y curl ncurses-dev u-boot-tools build-essential git vim libusb-1.0-0-dev pkg-config bc netpbm wget bzip2 debootstrap dpkg-dev qemu binfmt-support qemu-user-static apt-cacher
+RUN echo deb http://emdebian.org/tools/debian/ jessie main > /etc/apt/sources.list.d/emdebian.list
+RUN curl http://emdebian.org/tools/debian/emdebian-toolchain-archive.key | apt-key add -
+RUN dpkg --add-architecture armhf
 RUN apt-get update
-
-# Install packages for kernel and u-boot compilation
-RUN apt-get install --force-yes -y gcc-4.9-arm-linux-gnueabihf ncurses-dev uboot-mkimage build-essential git vim libusb-1.0-0-dev pkg-config bc netpbm wget tar bzip2
-
-RUN ln -s /usr/bin/arm-linux-gnueabihf-gcc-4.9 /usr/bin/arm-linux-gnueabihf-gcc
-
-# Install packages for debootstrap
-RUN apt-get install --force-yes -y debootstrap dpkg-dev qemu binfmt-support qemu-user-static dpkg-cross
+RUN apt-get install --force-yes -y crossbuild-essential-armhf