To Build Modules:

	make -C /lib/modules/`uname -r`/build M=${PWD} modules

To clean:

	make -C /lib/modules/`uname -r`/build M=${PWD} clean


The above commands starts by changing its directory to the one provided with the -C option (that is your kernel source directory)

There it finds the kernel's top level makefile. The M= option causes the Makefile to move back into your module source directory before trying to build the modules.

Note: M is not make option but argument passed to it

obj-m refers to the list of modules 

The kernel Makefile will read our Makefile to find out what to build, we specify that by writing obj-m += hello.o