[build] Move predefined all-drivers build shortcut to Makefile

The (very approximate) split between Makefile.housekeeping and
Makefile is that the former provides mechanism and the latter provides
policy.

Provide a section within Makefile as a home for predefined build
shortcuts such as the existing all-drivers build.

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

src/Makefile

 version :
 	@$(ECHO) "$(VERSION)"
 
+###############################################################################
+#
+# Predefined build shortcuts (for e.g. bin/ipxe.iso)
+
+# All drivers (excluding USB)
+#
+DRIVERS_ipxe	= $(DRIVERS_net) $(DRIVERS_infiniband) \
+		  $(DRIVERS_xen) $(DRIVERS_hyperv)
+
 ###############################################################################
 #
 # Drag in the bulk of the build system

src/Makefile.housekeeping

 	@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_ipxe	= $(DRIVERS_net) $(DRIVERS_infiniband) \
-		  $(DRIVERS_xen) $(DRIVERS_hyperv)
-
 # Analyse a target name (e.g. "bin/dfe538--prism2_pci.rom.tmp") and
 # derive the variables:
 #