memento_updater: Don't use absolute path for netboot environment.

When running under netboot (with busybox) the programs may be not in /bin, so we
should just use command name instead of specifying absolute path (e.g., date
instead of /bin/date).

BUG=none
TEST=none

Change-Id: I662468ad114c6174a25e31c1c3396aba0f403b15
Reviewed-on: https://chromium-review.googlesource.com/253961
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Wei-Ning Huang <wnhuang@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>
diff --git a/memento_updater_logging.sh b/memento_updater_logging.sh
index 4fb3540..598d48e 100755
--- a/memento_updater_logging.sh
+++ b/memento_updater_logging.sh
@@ -10,6 +10,6 @@
 
 log() {
   mkdir -p $(dirname $MEMENTO_AU_LOG)
-  echo $(/bin/date) "$*" >> "$MEMENTO_AU_LOG"
+  echo "$(date) $*" >>"$MEMENTO_AU_LOG"
 }
 export log