Fix UML hang on builder

The builder shuts off the STDIN while running the build script.
This impacts the UML because it needs some form of console to
write all the data out. Hence, define it's input/output channels.
Also parse the --builder option from build_test.sh so that the
script gracefully exits when running on the builder vs manual
mode that falls back to the UML shell.

BUG=23016816
TEST=build/build_test.sh < /dev/null

Change-Id: I7fbfcec690d0a7aa8440e9d6e49267b45f24f024