Let users provide their own startup.cfg without pain

Users can now edit base_image/startup.cfg without git paying attention
to it. Typing `make` and using the default works, too.

.gitignore

@@ -8,3 +8,4 @@
 **/icons.inc
 **/fox32os.img
 **/fox32os.img.tmp
+**/startup.cfg

Makefile

@@ -44,7 +44,11 @@ applications/launcher/icons.inc: applications/launcher/icons.png
 bootloader/bootloader.bin: bootloader/main.asm $(wildcard bootloader/*.asm)
 	$(FOX32ASM) $< $@
 
+base_image/startup.cfg: base_image/startup.cfg.default
+	cp $< $@
+
 FILES = \
+	base_image/startup.cfg \
 	base_image/kernel.fxf \
 	base_image/sh.fxf \
 	base_image/barclock.fxf \