drivers/mtd_default: add external declarations for mtd*

[gschorcht]

Contribution description

The mtd_default module defines MTD_NUMOF, if not present, based on the MTD_* defines. These defines are set to the corresponding mtd* MTD device pointer variables in the board definitions. However, not all mtd* MTD device pointer variables are always made known by external variable declarations. An example are SD Card Interfaces that are defined via the mtd_sdcard_default module. As a result, it may be necessary for an application using mtd_default to declare an external MTD device pointer variable mtd. To be able to use SD card for testing MTD based application, mtd_default also declares up to six mtd* MTD device pointer variables.

Testing procedure

Use any board without SD Card definition and compile tests/pkg_litllefs, tests/pkg_litllefs2 or tests/pkg_spiffs, for example to use a temporary connected SD Card interface.

CFLAGS='-DMTD_0=mtd0 -DCONFIG_USE_HARDWARE_MTD' USEMODULE='sdcard_spi mtd_sdcard_default' BOARD=nucleo-f411re make -j8 -C tests/pkg_littlefs

Without this PR, compilation fails due to undeclared mtd0 variable

tests/pkg_littlefs/bin/nucleo-f411re/riotbuild/riotbuild.h:2:15: error: 'mtd0' undeclared (first use in this function)
    2 | #define MTD_0 mtd0
      |               ^~~~

Compilation works with this PR and the temporary connected SD Card interface can be used with this test applications.

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

714b3e3 drivers/mtd_default: add external declarations for mtd*

Success	Failures	Total	Runtime
6931	0	6931	10m:37s

Artifacts

• Documentation preview

[benpicco]

bors merge

[bors]

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• Murdock
• static-tests
• tools-build-success

[gschorcht]

Thanks