build.sh: add documentation for TAGS_CONFIG env. var.

Change-Id: Ib150bf3f9212b533aaecadca4c62d1fedde7ef2b
Signed-off-by: Thierry Strudel <tstrudel@google.com>
diff --git a/build.sh b/build.sh
index 761b6c8..9d96084 100755
--- a/build.sh
+++ b/build.sh
@@ -76,6 +76,10 @@
 #   POST_KERNEL_BUILD_CMDS
 #     Command evaluated after `make`.
 #
+#   TAGS_CONFIG
+#     if defined, calls ./scripts/tags.sh utility with TAGS_CONFIG as argument
+#     and exit once tags have been generated
+#
 #   IN_KERNEL_MODULES
 #     if defined, install kernel modules
 #