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>

Whether or not we compile the com32/ code with DEBUG=1 we need to
provide the dprintf() implementation. This was highlighted by commit
72842b681e7b ("SEG(): make SEG() puke on bad pointers if DEBUG is
defined")

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

Add the ability to redirect dprintf to stdio when there is no other
choice.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Resolved Conflicts:

	com32/lib/dprintf.c
	com32/lib/vdprintf.c

Make dprintf() usable in the core, as it it on the mainline branch.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Fix an uninitialized pointer bug; return void rather than returning
int like normal printfs... if we're depending on the return value of a
debugging function we're screwed when debugging is disabled.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Fix incorrect prototype for vdprintf() used in dprintf(), masked by
the non-use of the header file.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Actually provide real infrastructure for debug-to-serial.  Very
useful when debugging fullscreen applications, especially under an
emulator like Qemu.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>