[build] Construct all-drivers list based on driver class

The USB bus drivers (ehci.c and xhci.c) have PCI device ID tables and
hence PCI_ROM() lines, but should probably not be included in the
all-drivers build on this basis, since they do nothing useful unless a
USB network driver is also present.

Fix by constructing the all-drivers list based on the driver class
(i.e. the portion of the source path immediately after "drivers/").

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

src/util/parserom.pl

@@ -161,7 +161,7 @@ sub print_make_rules {
     unless ( $state->{'is_header_printed'} ) {
         print "# NIC\t\n";
         print "# NIC\tfamily\t$state->{family}\n";
-        print "DRIVERS += $state->{driver_name}\n";
+        print "DRIVERS_$state->{driver_class} += $state->{driver_name}\n";
         print "\n";
         $state->{'is_header_printed'} = 1;
     }