Signed-off-by: Michael Brown <mcb30@etherboot.org>

For partly historical reasons, various files in the gPXE source tree
are licensed under different, though compatible, terms.  Most of the
code is licensed under GPLv2 with the "or later" clause, but there are
exceptions such as:

  The string.h file, which derives from Linux and is licensed as
  Public Domain.

  The EFI header files, which are taken from the EDK2 source tree and
  are licensed under BSD.

  The 3c90x driver, which has a custom GPL-like licence text.

Introduce a FILE_LICENCE() macro to make licensing more explicit.
This macro should be applied exactly once to each source (.c, .S or
.h) file.  It will cause a corresponding zero-sized common symbol to
be added to any .o files generated from that source file (and hence to
any final gPXE binaries generated from that source file).  Determining
the applicable licences to generated files can then be done using e.g.

  $ objdump -t bin/process.o | grep __licence
  00000000       O *COM*  00000001 .hidden __licence_gpl2_or_later

indicating that bin/process.o is covered entirely by the GPLv2
with the "or later" clause, or

  $ objdump -t bin/rtl8139.dsk.tmp | grep __licence
  00033e8c g     O .bss.textdata  00000000 .hidden __licence_gpl2_only
  00033e8c g     O .bss.textdata  00000000 .hidden __licence_gpl2_or_later
  00033e8c g     O .bss.textdata  00000000 .hidden __licence_public_domain

indicating that bin/rtl8139.dsk includes both code licensed under
GPLv2 (both with and without the "or later" clause) and code licensed
as Public Domain.

Determining the result of licence combinations is currently left as an
exercise for the reader.

GCC did not support #pragma GCC visibility until version 4.0.

Currently the only supported platform for x86_64 is EFI.

Building an EFI64 gPXE requires a version of gcc that supports
__attribute__((ms_abi)).  This currently means a development build of
gcc; the feature should be present when gcc 4.4 is released.

In the meantime; you can grab a suitable gcc tree from

  git://git.etherboot.org/scm/people/mcb30/gcc/.git

__cdecl is a misleading name, since it currently encapsulates both
cdecl and regparm(0) attributes.  Rename to __asmcall.

The usefulness of DBGLVL_IO is limited by the fact that many cards
require large numbers of uninteresting I/O reads/writes at device
probe time, typically when driving a bit-bashing I2C/SPI bus to read
the MAC address.

This patch adds the DBG_DISABLE() and DBG_ENABLE() macros, which can
be used to temporarily disable and re-enable selected debug levels.
Note that debug levels must still be enabled in the build in order to
function at all: you can't use DBG_ENABLE(DBGLVL_IO) in an object
built with DEBUG=object:1 and expect it to do anything.

Some drivers that still use the legacy-driver wrapper (tg3 in particular)
apparently do not specify their alignment constraints properly.  This
hack forces any __shared data to be maximally aligned.

Note that this provides only 16-byte alignment; it is not possible to
request alignment to any greater than 16 bytes using
__attribute__((aligned)), since the relocation code will preserve only 16
byte alignment (and operation under -DKEEP_IT_REAL cannot preserve more
that 16 byte alignment).

Idea proposed by Tim Hockin <thockin@google.com>

out-of-tree driver code.

out-of-tree driver code.

DBG_AC_IF(), to avoid namespace collisions.

while retaining the format string checking.

it is always used as __attribute__((...)).  Since variadic macros seem
to confuse poor doxygen, change it to a single-valued macro.

Remove compiler.h defines from osdep.h

to the object name in order to generate e.g. per-file variables.

DBG() several times to build up a line.