kbuild: allow to start building external modules in any directory

Unless an explicit O= option is provided, external module builds must
start from the kernel directory.

This can be achieved by using the -C option:

$ make -C /path/to/kernel M=/path/to/external/module

This commit allows starting external module builds from any directory,
so you can also do the following:

$ make -f /path/to/kernel/Makefile M=/path/to/external/module

The key difference is that the -C option changes the working directory
and parses the Makefile located there, while the -f option only
specifies the Makefile to use.

As shown in the examples in Documentation/kbuild/modules.rst, external
modules usually have a wrapper Makefile that allows you to build them
without specifying any make arguments. The Makefile typically contains
a rule as follows:

KDIR ?= /path/to/kernel
default:
$(MAKE) -C $(KDIR) M=$(CURDIR) $(MAKECMDGOALS)

The log will appear as follows:

$ make
make -C /path/to/kernel M=/path/to/external/module
make[1]: Entering directory '/path/to/kernel'
make[2]: Entering directory '/path/to/external/module'
CC [M] helloworld.o
MODPOST Module.symvers
CC [M] helloworld.mod.o
CC [M] .module-common.o
LD [M] helloworld.ko
make[2]: Leaving directory '/path/to/external/module'
make[1]: Leaving directory '/path/to/kernel'

This changes the working directory twice because the -C option first
switches to the kernel directory, and then Kbuild internally recurses
back to the external module directory.

With this commit, the wrapper Makefile can directly include the kernel
Makefile:

KDIR ?= /path/to/kernel
export KBUILD_EXTMOD := $(realpath $(dir $(lastword $(MAKEFILE_LIST))))
include $(KDIR)/Makefile

This avoids unnecessary sub-make invocations:

$ make
CC [M] helloworld.o
MODPOST Module.symvers
CC [M] helloworld.mod.o
CC [M] .module-common.o
LD [M] helloworld.ko

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>