setup-environment: clean up all variables

Add to the list of cleaned-up variables to include all the variables used by
this script.

Signed-off-by: Trevor Woerner <trevor.woerner@linaro.org>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
setup-environment: phraseology

Minor wording/grammar fixes which hopefully add clarity and make the resulting
text more agreeable.

Signed-off-by: Trevor Woerner <trevor.woerner@linaro.org>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
setup-environment: remove redundant information

In an error message, this informational text is irrelevant.

Signed-off-by: Trevor Woerner <trevor.woerner@linaro.org>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
setup-environment: improve MACHINE display

If an invalid machine is specified, do a better job at listing the supported
MACHINEs.

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

now:
Supported machines:
        cfa10036
        cfa10037
        cfa10049
        cfa10055
        cfa10056
        cfa10057
        cfa10058
        cgtqmx6
        imx233-olinuxino-maxi
        imx233-olinuxino-micro
        imx233-olinuxino-mini
        imx6qsabrelite
        m28evk
        m53evk
        nitrogen6x-lite
        nitrogen6x
        wandboard-dual
        wandboard-quad
        wandboard-solo
        imx23evk
        imx28evk
        imx31pdk
        imx35pdk
        imx51evk
        imx53ard
        imx53qsb
        imx6dlsabreauto
        imx6dlsabresd
        imx6qsabreauto
        imx6qsabresd
        imx6slevk
        imx6solosabreauto
        imx6solosabresd
        twr-vf65gs10

Signed-off-by: Trevor Woerner <trevor.woerner@linaro.org>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>