toolbox: add "uptime" command

Prints elapsed real time since boot, as well as idle time and sleep time.

Change-Id: I97f482d6087e9f802d74e91147bf767e6b2d4f42
Signed-off-by: Mike Lockwood <lockwood@android.com>
diff --git a/toolbox/Android.mk b/toolbox/Android.mk
index 70b13dc..bf7cfde 100644
--- a/toolbox/Android.mk
+++ b/toolbox/Android.mk
@@ -50,6 +50,7 @@
 	top \
 	iftop \
 	id \
+	uptime \
 	vmstat
 
 LOCAL_SRC_FILES:= \