[smc9000] Avoid using CONFIG as a preprocessor macro

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Allow for named configurations at build time

Allow named configurations to be specified via the CONFIG=... build
parameter.  For headers in config/*.h which support named
configurations, the following files will be included when building
with CONFIG=<name>:

  - config/defaults/<platform>.h (e.g. config/defaults/pcbios.h)

  - config/<header>.h

  - config/<name>/<header>.h (only if the directory config/<name> exists)

  - config/local/<header>.h (autocreated if necessary)

  - config/local/<name>/<header>.h (autocreated if necessary)

This mechanism allows for predefined named configurations to be
checked in to the source tree, as a directory config/<name> containing
all of the required header files.

The mechanism also allows for users to define multiple local
configurations, by creating header files in the directory
config/local/<name>.

Note that the config/*.h files which are used only to configure
internal iPXE APIs (e.g. config/ioapi.h) cannot be modified via a
named configuration.  This avoids rebuilding the entire iPXE codebase
whenever switching to a different named configuration.

Inspired-by: Robin Smidsrød <robin@smidsrod.no>
Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[intel] Display PBS value when applying ICH errata workaround

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[intel] Display before and after values for both PBS and PBA

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[intel] Apply PBS/PBA errata workaround only to ICH8 PCI device IDs

ICH8 devices have an errata which requires us to reconfigure the
packet buffer size (PBS) register, and correspondingly adjust the
packet buffer allocation (PBA) register.  The "Intel I/O Controller
Hub ICH8/9/10 and 82566/82567/82562V Software Developer's Manual"
notes for the PBS register that:

  10.4.20   Packet Buffer Size - PBS (01008h; R/W)

  Note: The default setting of this register is 20 KB and is
        incorrect. This register must be programmed to 16 KB.

  Initial value: 0014h
                 0018h (ICH9/ICH10)

It is unclear from this comment precisely which devices require the
workaround to be applied.  We currently attempt to err on the side of
caution: if we detect an initial value of either 0x14 or 0x18 then the
workaround will be applied.  If the workaround is applied
unnecessarily, then the effect should be just that we use less than
the full amount of the available packet buffer memory.

Unfortunately this approach does not play nicely with other device
drivers.  For example, the Linux e1000e driver will rewrite PBA while
assuming that PBS still contains the default value, which can result
in inconsistent values between the two registers, and a corresponding
inability to transmit or receive packets.  Even more unfortunately,
the contents of PBS and PBA are not reset by anything less than a
power cycle, meaning that this error condition will survive a hardware
reset.

The Linux driver (written and maintained by Intel) applies the PBS/PBA
errata workaround only for devices in the ICH8 family, identified via
the PCI device ID.  Adopt a similar approach, using the PCI_ROM()
driver data field to indicate when the workaround is required.

Reported-by: Donald Bindner <dbindner@truman.edu>
Debugged-by: Donald Bindner <dbindner@truman.edu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Add named configuration for VirtualBox

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