base: Declare a type for context IDs

Context IDs used to be declared as ad hoc (usually as int). This
changeset introduces a typedef for ContextIDs and a constant for
invalid context IDs.
diff --git a/src/cpu/minor/exec_context.hh b/src/cpu/minor/exec_context.hh
index 80d5d987..3e4ea5e 100644
--- a/src/cpu/minor/exec_context.hh
+++ b/src/cpu/minor/exec_context.hh
@@ -254,7 +254,7 @@
     unsigned int readStCondFailures() const { return 0; }
     void setStCondFailures(unsigned int st_cond_failures) {}
 
-    int contextId() { return thread.contextId(); }
+    ContextID contextId() { return thread.contextId(); }
     /* ISA-specific (or at least currently ISA singleton) functions */
 
     /* X86: TLB twiddling */