Documentation: update kbuild loadable modules goals & examples

Documentation: update kbuild loadable modules goals & examples

Update section 3.3 Loadable module goals - obj-m, from $(<module_name>-objs)
to $(<module_name>-y) for easier addition of conditional objects to the
module. The examples are also updated to reflect the current state of
each Makefile used.

Signed-off-by: matt mooney <[email protected]>
Reviewed-by: WANG Cong <[email protected]>
Signed-off-by: Randy Dunlap <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>