tool/create_iso: Make ISO creation tool configurable

Debian has dropped mkisofs long time ago, hence hardcoding it prevents
Genode from being built on Debian easily. This patch makes the tool
configurable (defaulting to mkisofs). Building on Debian can then be
done by something like:

ISOTOOL=genisoimage make run/foobar

Fixes #2234
Fixes #2235

tool/create_iso

@@ -4,9 +4,10 @@
 # \author  Christian Helmuth
 # \date    2009-02-05
 
-ISO    ?= genode
-ISODIR  = $(abspath $(ISO))
-ISOIMG  = $(abspath $(ISO).iso)
+ISO     ?= genode
+ISOTOOL ?= mkisofs
+ISODIR   = $(abspath $(ISO))
+ISOIMG   = $(abspath $(ISO).iso)
 
 default help:
 	@echo "--- available commands ---"
@@ -30,10 +31,10 @@ default help:
 # parameter 1  filename of ISO image
 # parameter 2  path of directory containing file tree for the ISO image
 #
-gen_iso_image = mkisofs     -f -l -R -hide-rr-moved -jcharset utf-8 \
-                            -no-emul-boot -boot-load-size 4 -boot-info-table \
-                            -b boot/isolinux/isolinux.bin \
-                            -o $(1) $(2)
+gen_iso_image = $(ISOTOOL) -f -l -R -hide-rr-moved -jcharset utf-8 \
+                           -no-emul-boot -boot-load-size 4 -boot-info-table \
+                           -b boot/isolinux/isolinux.bin \
+                           -o $(1) $(2)
 
 $(ISOIMG) iso:
 	@$(call gen_iso_image, $(ISOIMG) $(ISODIR))