linux: workaround make-4.1 bug

linux: workaround make-4.1 bug

On Ubuntu 18.04, make-4.1 emits spurious, incorrect "entering/leaving"
messages, which end up in the LINUX_VERSION_PROBED variable:

    printf 'probed linux version: "%s"\n' "$(LINUX_VERSION_PROBED)"
    probed linux version: "make[1]: Entering directory '/home/buildroot'
    4.19.78-linux4sam-6.2
    make[1]: Leaving directory '/home/buildroot/output/build/linux-linux4sam_6.2'"

First, the messages are displayed even though we do explicitly pass
--no-print-directory -s.

Second, the entering and leaving messages are not about the same
directory!

This *only* occurs in the following conditions:

  - the user has the correct 0022 umask,
  - top-level parallel is used (with or without PPD),
  - initial -C is specified as well.

    $ umask 0022
    $ make -j16 -C $(pwd)
    [...]
    depmod: ERROR: Bad version passed make[1]:
    [...]

(yes, 'make[1]:' is the string depmod is trying, and fails, to parse as
a version string).

If any of the three conditions above is removed, the problem no longer
occurs. Here's a table of the MAKEFLAGS:

                |                   0002                         |          0022            |
    ----+-------+------------------------------------------------+--------------------------+
        | no-j  | --no-print-directory --                        |                          |
    noC |       +------------------------------------------------+--------------------------+
        | -j16  | -j --jobserver-fds=3,4 --no-print-directory -- | -j --jobserver-fds=3,4   |
    ----+-------+------------------------------------------------+--------------------------+
        | no-j  | --no-print-directory --                        | w                        |
    -C  |       +------------------------------------------------+--------------------------+
        | -j16  | -j --jobserver-fds=3,4 --no-print-directory -- | w -j --jobserver-fds=3,4 |
    ----+-------+------------------------------------------------+--------------------------+

    0002: umask == 0002
    0022: umask == 0022

    no-j: no -j flag
    -j16: -j16 flag

    noC: no -C flag
    -C : -C /path/of/buildroot/

Only the bottom-right-most case fails...

This behaviour goes against what is documented:

    https://www.gnu.org/software/make/manual/make.html#g_t_002dw-Option

    5.7.4 The ‘--print-directory’ Option
    [...]
    you do not need to specify this option because ‘make’ does it for
    you: ‘-w’ is turned on automatically when you use the ‘-C’ option,
    and in sub-makes. make will not automatically turn on ‘-w’ if you
    also use ‘-s’, which says to be silent, or if you use
    ‘--no-print-directory’ to explicitly disable it.

So this exactly describes our situation; yet 'w' is added to MAKEFLAGS.

Getting rid of the 'w' flag makes the build succeed again, so that's
what we do here (bleark, icky)...

Furthermore, the documented way to override MAKEFLAGS is to do so as a
make parameter:

    https://www.gnu.org/software/make/manual/make.html#Options_002fRecursion

    5.7.3 Communicating Options to a Sub-make
    [...]
    If you do not want to pass the other flags down, you must change the
    value of MAKEFLAGS, like this:

        subsystem:
            cd subdir && $(MAKE) MAKEFLAGS=

However, doing so does not fix the issue. So we resort to pass the
modified MAKEFLAGS via the environment (bleark, icky)...

Fixes: #13141
Reported-by: Laurent <laurent@neko-labs.eu>
Reported-by: Asaf Kahlon <asafka7@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>