[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/Makefile.housekeeping

 #
 bobjs :
 	@$(ECHO) $(BOBJS)
+drivers_% :
+	@$(ECHO) $(DRIVERS_$*)
 drivers :
 	@$(ECHO) $(DRIVERS)
 .PHONY : drivers
 	@perl -ne 'chomp; print "$$1\n" if /\# NIC\t(.*)$$/' $^ >> $@
 CLEANUP		+= $(BIN)/NIC	# Doesn't match the $(BIN)/*.* pattern
 
+# Select drivers to be included in the all-drivers build
+#
+DRIVERS		= $(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:
 # 
 # TGT_DRIVERS  : the driver for each element (e.g. "rtl8139 prism2_pci")
 # TGT_ROM_NAME : the ROM name (e.g. "dfe538")
 #
-DRIVERS_ipxe	= $(DRIVERS)
 CARD_DRIVER	= $(firstword $(DRIVER_$(1)) $(1))
 TGT_ELEMENTS	= $(subst --, ,$(firstword $(subst ., ,$(notdir $@))))
 TGT_ROM_NAME	= $(firstword $(TGT_ELEMENTS))

src/arch/x86/Makefile

 
 # Include Hyper-V driver in the all-drivers build
 #
-DRIVERS += hyperv
+DRIVERS_hyperv += hyperv

src/arch/x86/Makefile.efi

 
 # Include SNP driver in the all-drivers build
 #
-DRIVERS += snp
+DRIVERS_net += snp
 
 # Rules for building EFI files
 #

src/util/parserom.pl

     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;
     }