scons: Add readCommandWithReturn helper

scons: Add readCommandWithReturn helper

In this way it will be possible to reliably catch any error
in the command execution which is not raising an exception
to Popen.

Change-Id: I4dc15648423f9bb8e8a470d97291dbd065c48eba
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28847
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>