Android.mk: clear variable at end of use

Clear fs_config_generate_extra_partition_list after use.

Test: build fs_config_files and fs_config_dirs targets.

Change-Id: Ie18bcd2df2ff1becb178e9ba45d226e2e9a8faab
Signed-off-by: William Roberts <william.c.roberts@intel.com>
diff --git a/tools/fs_config/Android.mk b/tools/fs_config/Android.mk
index dcd41aa..ab7f92d 100644
--- a/tools/fs_config/Android.mk
+++ b/tools/fs_config/Android.mk
@@ -305,6 +305,7 @@
 my_fs_config_h :=
 fs_config_generate_bin :=
 my_gen_oem_aid :=
+fs_config_generate_extra_partition_list :=
 
 # -----------------------------------------------------------------------------
 # Unit tests.