[build] Include ipxe.lkrn in default build

bin/ipxe.lkrn is built anyway in order to create bin/ipxe.iso, so
there is no additional cost to including it within the default build.

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

src/Makefile

 # Default build target: build the most common targets and print out a
 # helpfully suggestive message
 #
-ALL		:= bin/blib.a bin/ipxe.dsk bin/ipxe.iso bin/ipxe.usb \
-		   bin/undionly.kpxe bin/rtl8139.rom
+ALL		:= bin/blib.a bin/ipxe.dsk bin/ipxe.lkrn bin/ipxe.iso \
+		   bin/ipxe.usb bin/undionly.kpxe bin/rtl8139.rom
 all : $(ALL)
 	@$(ECHO) '==========================================================='
 	@$(ECHO)