boot/edk2: unset MAKEFLAGS in build environment

Yann reported in [1] that edk2 build could sometimes fail. The issue
can be reproduced when per-package directories is enabled, or also
when building on a system with GNU Make >= 4.4 using the
"--shuffle=reverse" option (such as Fedora 39). Those are pointing
toward a Makefile dependency issue.

The issue can be reproduced with commands:

    cat > .config <<EOF
    BR2_riscv=y
    BR2_RISCV_64=y
    BR2_TOOLCHAIN_EXTERNAL=y
    BR2_TARGET_EDK2=y
    EOF
    make olddefconfig

Then, building either with:

    make --shuffle=reverse

Or:

    utils/config -e BR2_PER_PACKAGE_DIRECTORIES
    make olddefconfig
    make -j$(nproc)

It is interesting to mention that when using "make --shuffle=reverse"
to build, the build can be completed if restarted only with "make". It
will not pull any other Buildroot package. This fact hints toward a
Makefile dependency issue internal to the EDK2 build system, rather
than in the Buildroot recipe.

The EDK2 build system is quite unique. See [2]. It generates files,
makefiles and internally uses GNU Make to compile code. This system is
likely not tested as being a sub-Make process in a complex Makefile
such as Buildroot.

In order to prevent Buildroot to pass unexpected Make flags to the
EDK2 sub-Make, this commit unset the MAKEFLAGS variable in the EDK2
build environment. This will put the EDK2 build script in a more
common and tested state. See GNU Make documentation about recursive use
of Make, more specifically [3].

Note: as mentioned, the build failure is likely due to an internal
issue of the EDK2 build system. The failure points to a missing
dependency in the EDK2 generator itself. This commit does not fix this
issue, but rather put the EDK2 build system in a normalized
environment, avoiding Buildroot flags being passed to the internal
EDK2 sub-Make invocation. The upstream EDK2 build system most likely
need a fix too.

Fixes:

    make[2]: *** No rule to make target '/buildroot/output/build/edk2-edk2-stable202308/Build/RiscVVirtQemu/RELEASE_GCC5/RISCV64/MdeModulePkg/Application/UiApp/UiApp/DEBUG/UiApp.efi', needed by '/buildroot/output/build/edk2-edk2-stable202308/Build/RiscVVirtQemu/RELEASE_GCC5/FV/Ffs/462CAA21-7614-4503-836E-8AB6F4662331UiApp/UiApp.offset'.  Stop.

    build.py...
     : error 7000: Failed to execute command
	    make tbuild [/buildroot/output/build/edk2-edk2-stable202308/Build/RiscVVirtQemu/RELEASE_GCC5/RISCV64/MdeModulePkg/Application/UiApp/UiApp]

    build.py...
     : error F002: Failed to build module
	    /buildroot/output/build/edk2-edk2-stable202308/MdeModulePkg/Application/UiApp/UiApp.inf [RISCV64, GCC5, RELEASE]

[1] https://lists.buildroot.org/pipermail/buildroot/2023-December/681507.html
[2] https://tianocore-docs.github.io/edk2-BuildSpecification/draft/4_edk_ii_build_process_overview/42_build_process_overview.html
[3] https://www.gnu.org/software/make/manual/make.html#Options_002fRecursion

Reported-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

boot/edk2/edk2.mk

@@ -150,7 +150,13 @@ EDK2_BASETOOLS_OPTS = \
 
 EDK2_PACKAGES_PATH = $(subst $(space),:,$(strip $(EDK2_PACKAGES_PATHS)))
 
+# EDK2 "build" script internally uses and calls "make", which controls
+# its own flags. It is mainly tested while not being a sub-make. In
+# order to stay in that configuration, we avoid leaking top-level
+# Buildroot make flags into EDK2 build by clearing the MAKEFLAGS
+# environment variable.
 EDK2_BUILD_ENV += \
+	MAKEFLAGS= \
 	WORKSPACE=$(@D) \
 	PACKAGES_PATH=$(EDK2_PACKAGES_PATH) \
 	PYTHON_COMMAND=$(HOST_DIR)/bin/python3 \