fix(common): Disable parallel_emerge by default for most commands.

I would like to phase out parallel_emerge so disable it for all commands
other than build_image which is the only one that shows a noticeable
benefit from it (~2 min with --fast, ~3 min with --nofast).

build_image

@@ -47,7 +47,7 @@ show_help_if_requested "$@"
 # not needed for the typical developer workflow.
 DEFINE_integer build_attempt 1 \
   "The build attempt for this image build."
-DEFINE_boolean fast "${DEFAULT_FAST}" \
+DEFINE_boolean fast ${FLAGS_TRUE} \
   "Use the parallel_emerge wrapper script."
 DEFINE_integer jobs "${NUM_JOBS}" \
   "How many packages to build in parallel at maximum."

common.sh

@@ -392,8 +392,8 @@ if [[ -f ${GCLIENT_ROOT}/src/scripts/.default_board ]]; then
   fi
 fi
 
-# Enable --fast by default.
-DEFAULT_FAST=${FLAGS_TRUE}
+# Disable --fast in most commands
+DEFAULT_FAST=${FLAGS_FALSE}
 
 # Directory to store built images.  Should be set by sourcing script when used.
 BUILD_DIR=

update_chroot

@@ -113,7 +113,7 @@ fi
 # In first pass, update portage and toolchains. Lagged updates of both
 # can cause serious issues later.
 info "Updating basic system packages"
-sudo -E ${EMERGE_CMD} ${EMERGE_FLAGS} \
+sudo -E ${EMERGE_CMD} --quiet ${EMERGE_FLAGS} \
     dev-util/ccache \
     sys-apps/portage \
     sys-devel/crossdev \