Generate PNG splash images from SVG files if available.

Also apply the fix in 382ef43c to arch_miniiso and arch_netboot_dir on
hurd.

build/config/hurd.cfg

@@ -63,6 +63,9 @@ arch_cd_info_dir:
 		cp $(GRUB_FONT) $(TEMP_CD_INFO_DIR)/boot/grub/font.pf2; \
 	fi
 	if [ -n "$(SPLASH_PNG)" ]; then \
+		if [ -n "$(SPLASH_SVG)" -a -f "$(SPLASH_SVG)" ]; then \
+			rsvg-convert $(SPLASH_SVG) > $(SPLASH_PNG); \
+		fi; \
 		cp $(SPLASH_PNG) $(TEMP_CD_INFO_DIR)/boot/grub/splash.png; \
 	fi
 
@@ -110,6 +113,9 @@ arch_netboot_dir:
 		cp $(GRUB_FONT) $(TEMP_NETBOOT_DIR)/$(NETBOOT_PATH)/font.pf2; \
 	fi
 	if [ -n "$(SPLASH_PNG)" ]; then \
+		if [ -n "$(SPLASH_SVG)" -a -f "$(SPLASH_SVG)" ]; then \
+			rsvg-convert $(SPLASH_SVG) > $(SPLASH_PNG); \
+		fi; \
 		cp $(SPLASH_PNG) $(TEMP_NETBOOT_DIR)/$(NETBOOT_PATH)/splash.png; \
 	fi