U-Boot: skip flash-image targets if no U-Boot image is passed

When building flash images we want a U-Boot binary for now.
Testing for it avoids dd waiting for input on stdin
instead leaving no clue.

src/system/boot/platform/u-boot/Jamfile

@@ -225,6 +225,9 @@ if $(HAIKU_BOARD_SDIMAGE_UBOOT_SCRIPT_NAME) {
 }
 
 # flash image targets
+# they currently depend on having an U-Boot binary available
+if $(ubootFile) {
+
 BuildUBootFlashImage haiku-$(HAIKU_BOOT_BOARD)_flash_image_raw.img : haiku_loader :
 	$(ubootFile) ;
 NotFile haiku-flash-image ;
@@ -240,6 +243,8 @@ BuildUBootFlashImage haiku-$(HAIKU_BOOT_BOARD)_flash_image_uimage.img : haiku_lo
 NotFile haiku-flash-uimage ;
 Depends haiku-flash-uimage : haiku-$(HAIKU_BOOT_BOARD)_flash_image_uimage.img ;
 
+}
+
 # SD/mmc image targets
 BuildUBootSDImage haiku-$(HAIKU_BOOT_BOARD).mmc : $(HAIKU_IMAGE) $(HAIKU_BOARD_SDIMAGE_FILES) ;
 NotFile haiku-mmc-image ;