build.sh: doc for PRE_DEFCONFIG_CMDS + POST_KERNEL_BUILD_CMDS

Change-Id: I42a138945c61040aa526220f2eb553f09c5c1842
Signed-off-by: Matthias Maennich <maennich@google.com>
diff --git a/build.sh b/build.sh
index fb69e13..aaf26a2 100755
--- a/build.sh
+++ b/build.sh
@@ -59,9 +59,15 @@
 #   SKIP_DEFCONFIG
 #     if defined, skip `make defconfig`
 #
+#   PRE_DEFCONFIG_CMDS
+#     Command evaluated before `make defconfig`
+#
 #   POST_DEFCONFIG_CMDS
 #     Command evaluated after `make defconfig` and before `make`.
 #
+#   POST_KERNEL_BUILD_CMDS
+#     Command evaluated after `make`.
+#
 #   IN_KERNEL_MODULES
 #     if defined, install kernel modules
 #