oeqa/runCmd: print stderr when that is a separate stream

oeqa/runCmd: print stderr when that is a separate stream

runCmd by default merges stderr into stdout, and only
needs to print stdout when errors occur. When stderr
is requested as a separate stream, and an error occurs,
stderr is discarded, obscuring useful error messages.
This changes the output to include both streams.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>