Improve instructions to build AGL Distribution

Improve instructions to build AGL Distribution

New build instructions are avairable.
 - Using 'repo' to download all repositories.
 - Only 1 shell script need to be run.

To install Repo:
    $ mkdir ~/bin
    $ export PATH=~/bin:$PATH

    $ curl https://storage.googleapis.com/git-repo-downloads/repo > ~/bin/repo
    $ chmod a+x ~/bin/repo

To build meta-agl/meta-agl-demo:
    $ repo init -u https://gerrit.automotivelinux.org/gerrit/AGL/AGL-repo
    $ repo sync
    $ source meta-agl/scripts/envsetup.sh <porter|qemux86-64|...> [build-dir]
    $ bitbake <target>

Note: currently AGL Distro supports only 2 machines, porter and qemux86-64.

Note for porter: Anyone who want to build for R-Car2 M2 Porter should
download Grahics Drivers and Multimedia Softwares from Renesas.
If not, envsetup.sh will stop and show some instruction, then
please follow it.

Change-Id: I1213130333e9a316bc3be5913cf2511c0cfb2853
Signed-off-by: Tadao Tanikawa <tanikawa.tadao@jp.panasonic.com>