backtrace: Extract rules from root Makefile

It's better to have them in a separate file instead of having them spread
across the Makefile. This is what the stack protector is already doing.

Change-Id: Id30742c0af10de5ea6d10674ca25bf52b0f2b262
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>