Fix out-of-tree build

<build>/ihk/cokernel/Makefile.common is not found when
<build>/mckernel/kernel/Makfile tries to perform
"Make -C <build>/ihk/{cokernel,ikc}" from mckernel/kernel

kernel/Makefile.in

@@ -9,7 +9,7 @@ V ?= $(VERBOSE)
 KERNEL = kernel.img
 KERNELS = $(addsuffix /$(KERNEL),$(addprefix $(O)/,$(BUILD_TARGET)))
 
-SUBCMD_OPTS = V='$(V)'
+SUBCMD_OPTS = V='$(V)' BUILD_IHK_COKERNEL=@abs_builddir@/../../ihk/cokernel
 
 $(if $(O),,$(error Specify the compilation target directory))
 #$(if $(shell ls $(IHKBASE)/Makefile),,\