Build apollo under ubuntu

建议在centos下编译apollo，没办法，只有自己搞定在Ubuntu下面的编译问题。
遇到的问题和解决方法如下：
1. no gmake
gmake 其实就是Ubuntu下面的make，只是个别系统如centos携带自身的make，所以用gmake指GNU make。
修改文件: pnx8400_a9_env.sh
----------------------------------------------------------------
# for most linux system such as Ubuntu, gmake is just make
if which gmake ; then
   export MAKE='gmake'
else
   export MAKE='make'
fi

if [ "$_TMTGTREL" = "retail" ]; then
   export MAKE="${MAKE} -s"
fi
------------------------------------------------------------------

2. bash string comparision
target/build/systems/linux/stbtv/makefile
------------------------------------------------------------------
check_config:
        @echo -n "Checking environment configuration ... "
        @if [ x$(NXP_BASE_ROOT) == x ] ; then \
                echo "Error: NXP_BASE_ROOT not set"; \
                exit 1; \
        fi
------------------------------------------------------------------
这样的语法会存在问题，需要把 == 改成 =进行字符串比较。
refer: http://tldp.org/LDP/abs/html/comparison-ops.html#EQUALSIGNREF