Build.sh changes to generate RPM inside centos docker.

I was having a few problems:

The BUILD_LOC sub folders: SPEC and SOURCES didn't exist. The tar commands didn't seem to like this.
I defined `_topdir` in build.sh rather than setting it as Readme.md suggests.

Entirely possible I was running the thing in a funny way though :confused: 

I was doing these commands in docker: 
```
RUN yum install -y rpm-build

RUN yum install -y git

RUN yum install -y wget

RUN yum install -y unzip
RUN yum install -y glibc.i686

RUN git clone https://github.com/jasraj/q-build.git


RUN wget -P /tmp/ http://kx.com/347_d0szre-fr8917_llrsT4Yle-5839sdX/3.1/linux.zip 

RUN unzip -d /tmp /tmp/linux.zip 

RUN q-build/rpm/build.sh 3.1-2014.08.22 /tmp/q /tmp/rpm-output
```