Update external/Makefile to remove object files.
Update external/Makefile to remove object files.

To resolve the issue #186

Change-Id: I111afa188ac0463614fecd442efe7f1c3d1de7dc
Signed-off-by: Heejin Kim <297a49b856454238e481497743c9f1755c937548@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT