[build] Fix .ids.o creation for drivers not in the all-drivers build

Commit dc19e63 ("[build] Construct all-drivers list based on driver
class") accidentally excluded the USB bus drivers from the list of
files parsed in order to create PCI 3.0 device ID lists.

Fix by returning $(DRIVERS) to its previous definition as a list of
all driver files, and use only $(DRIVERS_ipxe) to contain the
filtered list containing only those drivers which we want to include
in the "all-drivers" build.

Reported-by: Mary-Ann Johnson <MaryAnn.Johnson@displaylink.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 
 # Select drivers to be included in the all-drivers build
 #
-DRIVERS		= $(DRIVERS_net) $(DRIVERS_infiniband) \
+DRIVERS_ipxe	= $(DRIVERS_net) $(DRIVERS_infiniband) \
 		  $(DRIVERS_xen) $(DRIVERS_hyperv)
-DRIVERS_ipxe	= $(DRIVERS)
 
 # Analyse a target name (e.g. "bin/dfe538--prism2_pci.rom.tmp") and
 # derive the variables:

src/util/parserom.pl

         print "# NIC\t\n";
         print "# NIC\tfamily\t$state->{family}\n";
         print "DRIVERS_$state->{driver_class} += $state->{driver_name}\n";
+        print "DRIVERS += $state->{driver_name}\n";
         print "\n";
         $state->{'is_header_printed'} = 1;
     }