Only compile dprintf/vdprintf if DEBUG_PORT is defined

Only compile dprintf/vdprintf if DEBUG_PORT is defined

We really, really don't want to accidentally spew output to a debug
port in production, so make it obligatory to define DEBUG_PORT or
DEBUG_STDIO in order for the debugging code to be compiled in.  Since
DEBUG_STDIO just turns the debugging code into stdio references, we
only need to compile the code for the DEBUG_PORT case.

Add a commented-out line to mk/devel.mk to make it easier for users.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Matt Fleming <matt.fleming@intel.com>