[build] Search for ldlinux.c32 separately from isolinux.bin

Some distributions (observed with Ubuntu 15.04) place ldlinux.c32 in a
separate directory from isolinux.bin.  Search for these files
separately, and allow an alternative location of ldlinux.c32 to be
provided via LDLINUX_C32=... on the make command line.

Reported-by: Adrian Koshka <adriankoshcha@teknik.io>
Tested-by: Adrian Koshka <adriankoshcha@teknik.io>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/Makefile

 #
 CFLAGS_lkrnprefix	+= -DVERSION="\"$(VERSION)\""
 
-# Locations of utilities
-#
+# Locations of isolinux files
+#
+SYSLINUX_DIR_LIST	:= \
+	/usr/lib/syslinux \
+	/usr/lib/syslinux/bios \
+	/usr/lib/syslinux/modules/bios \
+	/usr/share/syslinux \
+	/usr/share/syslinux/bios \
+	/usr/share/syslinux/modules/bios \
+	/usr/local/share/syslinux \
+	/usr/local/share/syslinux/bios \
+	/usr/local/share/syslinux/modules/bios \
+	/usr/lib/ISOLINUX
 ISOLINUX_BIN_LIST	:= \
 	$(ISOLINUX_BIN) \
-	/usr/lib/syslinux/isolinux.bin \
-	/usr/lib/syslinux/bios/isolinux.bin \
-	/usr/share/syslinux/isolinux.bin \
-	/usr/share/syslinux/bios/isolinux.bin \
-	/usr/local/share/syslinux/isolinux.bin \
-	/usr/local/share/syslinux/bios/isolinux.bin \
-	/usr/lib/ISOLINUX/isolinux.bin
+	$(patsubst %,%/isolinux.bin,$(SYSLINUX_DIR_LIST))
+LDLINUX_C32_LIST	:= \
+	$(LDLINUX_C32) \
+	$(patsubst %,%/ldlinux.c32,$(SYSLINUX_DIR_LIST))
 ISOLINUX_BIN	= $(firstword $(wildcard $(ISOLINUX_BIN_LIST)))
+LDLINUX_C32	= $(firstword $(wildcard $(LDLINUX_C32_LIST)))
 
 # i386-specific directories containing source files
 #

src/arch/i386/Makefile.pcbios

 NON_AUTO_MEDIA	+= iso
 %iso:	%lkrn util/geniso
 	$(QM)$(ECHO) "  [GENISO] $@"
-	$(Q)ISOLINUX_BIN=$(ISOLINUX_BIN) VERSION="$(VERSION)" bash util/geniso -o $@ $<
+	$(Q)ISOLINUX_BIN=$(ISOLINUX_BIN) LDLINUX_C32=$(LDLINUX_C32) \
+	    VERSION="$(VERSION)" bash util/geniso -o $@ $<
 
 # rule to make a floppy emulation ISO boot image
 NON_AUTO_MEDIA	+= liso

src/util/geniso

 		cp ${ISOLINUX_BIN} ${dir}
 
 		# syslinux 6.x needs a file called ldlinux.c32
-		LDLINUX_C32=$(dirname ${ISOLINUX_BIN})/ldlinux.c32
-		if [ -s ${LDLINUX_C32} ]; then
+		if [ -n "${LDLINUX_C32}" -a -s "${LDLINUX_C32}" ]; then
 			cp ${LDLINUX_C32} ${dir}
 		fi