samd/mcu/samd51: Enable FAT support for SAMD51.

Tested with a SD card connected to a SAMD51 board.  The SEEED WIO terminal
has a SD-Card reader built-in.

Also a side change to remove a few obsolete lines from Makefile.

ports/samd/Makefile

@@ -61,7 +61,6 @@ CFLAGS_MCU_SAMD51 = -mtune=cortex-m4 -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-a
 CFLAGS += $(INC) -Wall -Werror -std=c99 -nostdlib -mthumb $(CFLAGS_MCU_$(MCU_SERIES)) -fsingle-precision-constant -Wdouble-promotion
 CFLAGS += -DMCU_$(MCU_SERIES) -D__$(CMSIS_MCU)__
 CFLAGS += $(CFLAGS_EXTRA)
-CFLAGS += -DMPCONFIG_MCU_H='<boards/mpconfig_$(MCU_SERIES_LOWER).h>'
 
 LDFLAGS += -nostdlib $(addprefix -T,$(LD_FILES)) -Map=$@.map --cref
 
@@ -169,9 +168,6 @@ CFLAGS += -DMICROPY_MODULE_FROZEN_STR
 CFLAGS += -DMICROPY_QSTR_EXTRA_POOL=mp_qstr_frozen_const_pool
 endif
 
-# Workaround for bug in older gcc, warning on "static usbd_device_t _usbd_dev = { 0 };"
-$(BUILD)/lib/tinyusb/src/device/usbd.o: CFLAGS += -Wno-missing-braces
-
 all: $(BUILD)/firmware.uf2
 
 $(BUILD)/firmware.elf: $(OBJ)

ports/samd/fatfs_port.c

@@ -0,0 +1,41 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2013, 2014 Damien P. George
+ * Copyright (c) 2021 Robert Hammelrath
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "mphalport.h"
+#include "py/runtime.h"
+#include "shared/timeutils/timeutils.h"
+#include "lib/oofatfs/ff.h"
+
+extern uint32_t time_offset;
+
+MP_WEAK DWORD get_fattime(void) {
+    timeutils_struct_time_t tm;
+
+    timeutils_seconds_since_epoch_to_struct_time(mp_hal_ticks_ms_64() / 1000 + time_offset, &tm);
+    return ((tm.tm_year - 1980) << 25) | ((tm.tm_mon) << 21) | ((tm.tm_mday) << 16) |
+           ((tm.tm_hour) << 11) | ((tm.tm_min) << 5) | (tm.tm_sec / 2);
+}

ports/samd/mcu/samd51/mpconfigmcu.h

@@ -22,6 +22,12 @@ unsigned long trng_random_u32(void);
 // Due to a limitation in the TC counter for us, the ticks period is 2**29
 #define MICROPY_PY_UTIME_TICKS_PERIOD   (0x20000000)
 
+// fatfs configuration used in ffconf.h
+#define MICROPY_FATFS_ENABLE_LFN            (1)
+#define MICROPY_FATFS_RPATH                 (2)
+#define MICROPY_FATFS_MAX_SS                (4096)
+#define MICROPY_FATFS_LFN_CODE_PAGE         437 /* 1=SFN/ANSI 437=LFN/U.S.(OEM) */
+
 #define VFS_BLOCK_SIZE_BYTES            (1536) //
 
 #define MICROPY_HW_UART_TXBUF           (1)

ports/samd/mcu/samd51/mpconfigmcu.mk

@@ -1,8 +1,11 @@
 MICROPY_VFS_LFS2 ?= 1
+MICROPY_VFS_FAT ?= 1
 
 SRC_S += shared/runtime/gchelper_m3.s
 
-SRC_C += drivers/dht/dht.c \
+SRC_C += \
+	fatfs_port.c \
+	drivers/dht/dht.c \
 
 LIBM_SRC_C +=  $(addprefix lib/libm/,\
 	acoshf.c \

ports/samd/modutime.c

@@ -29,7 +29,7 @@
 #include "shared/timeutils/timeutils.h"
 #include "mphalport.h"
 
-static uint32_t time_offset = 0;
+uint32_t time_offset = 0;
 
 // localtime([secs])
 STATIC mp_obj_t time_localtime(size_t n_args, const mp_obj_t *args) {