Set HAVE_UNWIND_CONTEXT_STRUCT for Clang

Change-Id: I423d650181c6736111e4a5e91ded81c471dd08e1
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
SignApk: update SignApk to support SHA256withRSA signing

(cherry picked from commit c0581a02ede48007623030c4ce13174ee972cc42)

Change-Id: I6212e0f3a6aa3265b111f0f7348b667af4a668b0
SignApk: add support for EC keys

Change-Id: I09bc0e52fc84ec321b997016a8f97de1bfd2770d