base: Add a static assert to check bit union ranges

If a bit field in a bit union specified as Bitfield<LSB, MSB> instead
of Bitfield<MSB, LSB> the code silently fails and the field is read as
zero. This changeset introduces a static assert that tests, at compile
time, that the bit order is correct.

src/base/bitunion.hh

@@ -85,6 +85,9 @@ namespace BitfieldBackend
         template<int first, int last=first>
         class Bitfield : public BitfieldBase<Type>
         {
+            static_assert(first >= last,
+                          "Bitfield ranges must be specified as <msb, lsb>");
+
           public:
             operator const uint64_t () const
             {