[debug] Allow debug message colours to be customised via DBGCOL=...

When multiple iPXE binaries are running concurrently (e.g. in the case
of undionly.kpxe using an underlying iPXE driver via the UNDI
interface) it would be helpful to be able to visually distinguish
debug messages from each binary.

Allow the range of debug colours used to be customised via the
DBGCOL=...  build parameter.  For example:

  # Restrict to colours 31-33 (red, green, yellow)
  make DBGCOL=31-33

  # Restrict to colours 34-36 (blue, magenta, cyan)
  make DBGCOL=34-36

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Expose build timestamp, build name, and product names

Expose the build timestamp (measured in seconds since the Epoch) and
the build name (e.g. "rtl8139.rom" or "ipxe.efi"), and provide the
product name and product short name in a single centralised location.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Allow device paths to be easily included in debug messages

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Provide a meaningful EFI SNP device name

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Restructure EFI driver model

Provide a single instance of EFI_DRIVER_BINDING_PROTOCOL (attached to
our image handle); this matches the expectations scattered throughout
the EFI specification.

Open the underlying hardware device using EFI_OPEN_PROTOCOL_BY_DRIVER
and EFI_OPEN_PROTOCOL_EXCLUSIVE, to prevent other drivers from
attaching to the same device.

Do not automatically connect to devices when being loaded as a driver;
leave this task to the platform firmware (or to the user, if loading
directly from the EFI shell).

When running as an application, forcibly disconnect any existing
drivers from devices that we want to control, and reconnect them on
exit.

Provide a meaningful driver version number (based on the build
timestamp), to allow platform firmware to automatically load newer
versions of iPXE drivers if multiple drivers are present.

Include device paths within debug messages where possible, to aid in
debugging.

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