bytestring: Add initializer macro

bytestring: Add initializer macro

The BYTESTRING macro is constant, since it's designed to take a string
literal, but it doesn't count as constant since it involves an (inlined)
function call.  That means it can't be used in a static storage duration
initializer.

Unfortunately, I can't see a portable way to make something which can be
used as an initializer, but which can also be used in other contexts where
BYTESTRING() can be used, so this patch introduces BYTESTRING_INIT() which
is similar to BYTESTRING() but produces an initializer.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>