Explain the Makefile generated by mkmakefile (Linux kernel, buildroot, busybox)

Question

The script mkmakefile generates the following Makefile in the builddir.

If invoked all targets are passed to the srcdir Makefile.

Can anyone explain why? Where is the magic?

lastword = $(word $(words $(1)),$(1))
makedir := $(dir $(call lastword,\$(MAKEFILE_LIST)))

MAKEARGS := -C path_to_srcdir
MAKEARGS += O=$(if $(patsubst /%,,$(makedir)),$(CURDIR)/)$(patsubst %/,%,$(makedir))

MAKEFLAGS += --no-print-directory

.PHONY: all $(MAKECMDGOALS)

all := $(filter-out all Makefile,$(MAKECMDGOALS))

all:
    $(MAKE) $(MAKEARGS) $(all)

Makefile:;

$(all): all
    @:

%/: all
    @:

Answer 1

Let's break it down. First, using all as both a target and a variable name is just obnoxious, something a novice coder or a code generator would do. Let's rename the variable all_targs and assume we're invoking by make Makefile foo bar baz. If we scratch a few parts that aren't relevant to the question, we get

MAKEARGS := -C path_to_srcdir

.PHONY: all $(MAKECMDGOALS)

all_targs := $(filter-out all Makefile,$(MAKECMDGOALS)) # foo bar baz

all:
    $(MAKE) $(MAKEARGS) $(all_targs)

Makefile:;

$(all_targs): all
    @: # do nothing

So we do nothing Makefile, all_targs is foo bar baz, and for those three targets we do nothing but we require all. The rule for all runs the command

make -C path_to_srcdir foo bar baz

(There are a few more twists, but that's the core of it.)