Parameterize the kernel module build dir in all makefiles.

Builds that can target kernels besides the currently running one cannot
work when the path is hard-coded to use (uname -r).
Define KDIR parameter and set a default value if it is undefined.

Author: nathansgreen

example-sysctl/Makefile

@@ -1,9 +1,10 @@
 obj-m := examplesysctl.o
 examplesysctl-objs := target/x86_64-linux-kernel-module/debug/libexample_sysctl.a
 EXTRA_LDFLAGS += --entry=init_module
+KDIR ?= /lib/modules/$(shell uname -r)/build
 
 all:
-	$(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(CURDIR)
+	$(MAKE) -C $(KDIR) M=$(CURDIR)
 
 clean:
-	$(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(CURDIR) clean
+	$(MAKE) -C $(KDIR) M=$(CURDIR) clean

hello-world/Makefile

@@ -1,9 +1,10 @@
 obj-m := helloworld.o
 helloworld-objs := target/x86_64-linux-kernel-module/debug/libhello_world.a
 EXTRA_LDFLAGS += --entry=init_module
+KDIR ?= /lib/modules/$(shell uname -r)/build
 
 all:
-	$(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(CURDIR)
+	$(MAKE) -C $(KDIR) M=$(CURDIR)
 
 clean:
-	$(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(CURDIR) clean
+	$(MAKE) -C $(KDIR) M=$(CURDIR) clean

static-filesystem/Makefile

@@ -1,9 +1,10 @@
 obj-m := staticfilesystem.o
 staticfilesystem-objs := target/x86_64-linux-kernel-module/debug/libstatic_filesystem.a
 EXTRA_LDFLAGS += --entry=init_module
+KDIR ?= /lib/modules/$(shell uname -r)/build
 
 all:
-	$(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(CURDIR)
+	$(MAKE) -C $(KDIR) M=$(CURDIR)
 
 clean:
-	$(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(CURDIR) clean
+	$(MAKE) -C $(KDIR) M=$(CURDIR) clean

tests/Makefile

@@ -1,9 +1,10 @@
 obj-m := testmodule.o
 testmodule-objs := $(TEST_LIBRARY)
 EXTRA_LDFLAGS += --entry=init_module
+KDIR ?= /lib/modules/$(shell uname -r)/build
 
 all:
-	$(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(CURDIR)
+	$(MAKE) -C $(KDIR) M=$(CURDIR)
 
 clean:
-	$(MAKE) -C /lib/modules/$(shell uname -r)/build M=$(CURDIR) clean
+	$(MAKE) -C $(KDIR) M=$(CURDIR) clean