stubdom: fix build dependencies

diff -r e80d27b6e206 stubdom/Makefile
@@ -232,7 +232,7 @@
 ######
 
 .PHONY: caml
-caml: cross-newlib
+caml: $(CROSS_ROOT)
 	CPPFLAGS="$(TARGET_CPPFLAGS)" CFLAGS="$(TARGET_CFLAGS)" $(MAKE) -C $@ LWIPDIR=$(CURDIR)/lwip 
 
 ###

@@ -240,7 +240,7 @@
 ###
 
 .PHONY: c
-c: cross-newlib
+c: $(CROSS_ROOT)
 	CPPFLAGS="$(TARGET_CPPFLAGS)" CFLAGS="$(TARGET_CFLAGS)" $(MAKE) -C $@ LWIPDIR=$(CURDIR)/lwip 
 
 ######

@@ -258,7 +258,7 @@
 	done
 
 .PHONY: grub
-grub: grub-upstream cross-newlib
+grub: grub-upstream $(CROSS_ROOT)
 	CPPFLAGS="$(TARGET_CPPFLAGS)" CFLAGS="$(TARGET_CFLAGS)" $(MAKE) -C $@
 
 ########