setup-environment: add options for the script

Add options to the script machine, below are the options added:

. ./setup-environment

Usage: source setup-environment <build-dir>
    <build-dir>: required option; specifies build directory location

By default the script will setup MACHINE to be imx6qsabresd.

Supported machines: imx23evk imx28evk imx31pdk imx35pdk imx51evk
                    imx53ard imx53qsb imx6dlsabresd imx6qsabreauto
                    imx6qsabresd imx6slevk cgtqmx6
                    imx233-olinuxino-maxi imx233-olinuxino-micro
                    imx233-olinuxino-mini imx6qsabrelite nitrogen6x
                    wandboard-dual wandboard-solo

To build for a machine listed above, run this script as:
MACHINE=<machine> source setup-environment <build-dir>

The script sets PARALLEL_MAKE & BB_NUMBER_THREADS to 4

Change-Id: I192f006a323fe99cccfb32371ee426b104d9598b
Signed-off-by: Mahesh Mahadevan <Mahesh.Mahadevan@freescale.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>