Handle partitioned and unpartitioned SD cards

Add utils.MBRPartition to implement basic partitioned device support.

Try to mount partition 1 of SDCard first, if that fails, try to mount
entire device as FAT file system.

Signed-off-by: Matthias Blankertz <matthias@blankertz.org>

software/src/utils/sdcontext.py

@@ -0,0 +1,39 @@
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2025 Matthias Blankertz <matthias@blankertz.org>
+
+import os
+
+from . import MBRPartition
+from rp2_sd import SDCard
+
+
+class SDContext:
+    def __init__(self, mosi, miso, sck, ss, baudrate):
+        self.mosi = mosi
+        self.miso = miso
+        self.sck = sck
+        self.ss = ss
+        self.baudrate = baudrate
+
+    def __enter__(self):
+        self.sdcard = SDCard(self.mosi, self.miso, self.sck, self.ss, self.baudrate)
+        # Try first partition
+        try:
+            self.part = MBRPartition(self.sdcard, 0)
+            os.mount(self.part, '/sd')
+            return self
+        except Exception:
+            print("Failed to mount SDCard partition, trying whole device...")
+        # Try whole device
+        try:
+            os.mount(self.sdcard, '/sd')
+            return self
+        except Exception as ex:
+            self.sdcard.deinit()
+            raise RuntimeError("Could not mount SD card") from ex
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        try:
+            os.umount('/sd')
+        finally:
+            self.sdcard.deinit()