Add eMMC boot partition dump support (boot0/boot1)

Extends the emmc_server payload to access the eMMC hardware boot
partitions (boot0/boot1) that hold the Tegra T124 BCT and low-level
bootloader — data not captured in the existing NAND dump.

Payload changes (emmc_server.c / emmc_server.h):
- Add MMC_CMD6 (SWITCH, R1b) for EXT_CSD partition switching
- Add switch_partition_access() to set EXT_CSD[179] PARTITION_CONFIG
- EMMC_CMD_READ and EMMC_CMD_WRITE now decode high 2 bits of
  start_sector to select the target partition without new op codes:
    0x80000000|sector -> boot0, 0xC0000000|sector -> boot1
  Partition is restored to UDA after each operation.

New script (dump_boot_partitions.py):
- Reads EXT_CSD BOOT_SIZE_MULT to determine exact partition size
- Dumps jibo_work/emmc_boot0.bin and jibo_work/emmc_boot1.bin
- Works with the existing shofel2_t124 EMMC_READ command unchanged

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Shofel/include/emmc_server.h

@@ -15,14 +15,22 @@
 #define EMMC_CMD_ERASE          0x05  /* Erase a range of sectors (forces reallocation) */
 #define EMMC_CMD_EXIT           0xFF
 
-/* Transfer chunk sizes basically the amount much data is sent/received per USB transfer was not multiple of 0x1000
- *
- * Reads: 8 sectors (4KB) = 1.1 MB/s - OPTIMIZED
- * Writes: 1 sector (512B) - ORIGINAL (safe)
+/* Boot partition encoding for EMMC_CMD_READ / EMMC_CMD_WRITE start_sector:
+ *   0x00000000 | sector  -> User Data Area  (normal)
+ *   0x80000000 | sector  -> Boot Partition 1 (boot0)
+ *   0xC0000000 | sector  -> Boot Partition 2 (boot1)
+ * The payload switches the eMMC partition, performs the I/O, then restores UDA.
+ * Use sector=0 and num_sectors=boot_size_sectors to dump an entire boot partition.
+ */
+#define EMMC_BOOT0_BASE  0x80000000U
+#define EMMC_BOOT1_BASE  0xC0000000U
+
+/* Transfer chunk sizes: sectors per USB transfer.
+ * Both read and write now use 8-sector (4 KB) chunks via CMD18/CMD25.
  */
 #define EMMC_CHUNK_SECTORS_READ    8
-#define EMMC_CHUNK_SECTORS_WRITE   1
-#define EMMC_CHUNK_SECTORS         1   /* Default - single sector for compatibility */
+#define EMMC_CHUNK_SECTORS_WRITE   8
+#define EMMC_CHUNK_SECTORS         8
 #define EMMC_SECTOR_SIZE           512
 #define EMMC_CHUNK_BYTES           (EMMC_CHUNK_SECTORS * EMMC_SECTOR_SIZE)