Android: clean only objects, not the folders

Some users building with a high number of threads
reported to me problems regarding the import_includes folder.

So, only delete objects files (.o) and keep import_includes

Change-Id: Ibf3ac818ff2bcf00bdf4f5243112001e2b2e15ef
Android: typo in busybox folder

That was a remains of a test when the folder doesnt exists

Change-Id: If1989dc774ae7c6a8a94692ada6c321f67b5f4d9