On a Dell OptiPlex 7010, calling DisconnectController() on the LOM
device handle will lock up the system.  Debugging shows that execution
is trapped in an infinite loop that is somehow trying to reconnect
drivers (without going via ConnectController()).

The problem can be reproduced in the UEFI shell with no iPXE code
present, by using the "disconnect" command.  Experimentation shows
that the only fix is to unload (rather than just disconnect) the
"Ip4ConfigDxe" driver.

Add the concept of a blacklist of UEFI drivers that will be
automatically unloaded when iPXE runs as an application, and add the
Dell Ip4ConfigDxe driver to this blacklist.

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