setup-environment: return if no conf/local.conf generated

there will be no conf/local.conf generated if python version check
in poky/scripts/oe-buildenv-internal failed. In this case, no need
to go further.

Signed-off-by: Ting Liu <ting.liu@freescale.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
setup-environment: check script has been sourced

Check that this script has been invoked correctly (i.e. that it has been
"source"d and not merely run directly). If it has been run directly, don't
exit immediately in case the user specified the "help" option, allow the help
to be displayed, then exit. Adjust how this script is terminated based on
whether it has been sourced or run directly (i.e. use either "return" or
"exit" as required).

This change fixes an infinite loop that is caused if the user runs this script
directly and specifies the help option.

These changes have been tested on: bash, dash, and zsh.

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

Before trying to show the supported MACHINEs, make sure they can be found,
otherwise the error message to "ls" will be displayed (which users might find
confusing).

Tested on: bash, dash, zsh.

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