Fix missing realloc function in SI

Fix missing realloc function in SI

The realloc payload function was missing in the SI port.

This change also adds the ability to build and run SI95
test applications in the test_app directory. NNG tests
are still run by default (this will be changed eventually).

Signed-off-by: E. Scott Daniels <daniels@research.att.com>
Change-Id: Ie8a453a009767afcf23eac610dc5f976b880d0aa