This commit removes the Makefile-level MICROPY_FATFS config and moves the
MICROPY_VFS_FAT config to the Makefile level to replace it. It also moves
the include of the oofatfs source files in the build from each port to a
central place in extmod/extmod.mk.
For a port to enabled VFS FAT support it should now set MICROPY_VFS_FAT=1
at the level of the Makefile. This will include the relevant oofatfs files
in the build and set MICROPY_VFS_FAT=1 at the C (preprocessor) level.
Setting MICROPY_PY_USSL and MICROPY_SSL_MBEDTLS at the Makefile-level will
now build mbedTLS from source and include it in the build, with the ussl
module using this TLS library. Extra settings like MBEDTLS_CONFIG_FILE may
need to be provided by a given port.
If a port wants to use its own mbedTLS library then it should not set
MICROPY_SSL_MBEDTLS at the Makefile-level but rather set it at the C level,
and provide the library as part of the build in its own way (see eg esp32
port).
Damien George