Update user guide

Following Out of Box testing for v1.5 release:

    Update host OS version to Ubuntu 16.04
    Clarify configuration files needed for checkpatch
    Add note on using Linaro precompiled binaries

Change-Id: Ia4ae61e01128ddff1a288972ddf84b79370fa52c
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>