Add modifier goal incrementaljavac

With "incrementaljavac" on your make command line, you will recompile
only Java source files that have been changed since last build.
Nothing is changed if "incrementaljavac" is not among your make goals.

Change-Id: Iae591459827df81ac91e95292464db5e9828343a
diff --git a/envsetup.sh b/envsetup.sh
index 4b2c19e..2faeed1 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -695,6 +695,8 @@
                     ARGS="$ARGS showcommands"
                 elif [ "$DIR" = dist ]; then
                     ARGS="$ARGS dist"
+                elif [ "$DIR" = incrementaljavac ]; then
+                    ARGS="$ARGS incrementaljavac"
                 else
                     echo "No Android.mk in $DIR."
                     return 1