terminal.py: Restore error output from Terminal

terminal.py: Restore error output from Terminal

In bitbake commit 1ecc1d94 (process: Do not mix stderr with stdout),
bb.process.Popen() was changed to no longer combine stdout and stderr by
default. However, the Terminal class was not updated to reflect this and
subsequently only output stdout in case of failures.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>