chore(core/tools): add decomobine snippet, add readme

core/tools/snippets/README.md

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+# Core-specific snippets
+
+## [decombine.py](decombine.py)
+
+Take a `combined.bin` file which is the output of `core/tools/combine_firmware`,
+split it back into original parts, and verify that there is no unnaccounted for noise.
+
+## [change_icon_format.py](change_icon_format.py)
+
+Converts all TOIF icons from the old endianity to the new one.

core/tools/snippets/decombine.py

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+import io
+import sys
+from pathlib import Path
+from trezorlib._internal import firmware_headers
+from trezorlib.firmware.core import FirmwareHeader, HeaderType
+
+INFILE = Path(sys.argv[1])
+DATA = INFILE.read_bytes()
+
+READER = io.BytesIO(DATA)
+
+# boardloader is first 3 16kB sectors
+BOARDLOADER = READER.read(3 * 16 * 1024)
+
+# following 16kB is unused
+UNUSED = READER.read(16 * 1024)
+if all(b == 0 for b in UNUSED):
+    print("Unused space is all zero")
+elif all(b == 0xFF for b in UNUSED):
+    print("Unused space is all 0xFF")
+else:
+    print("WARNING: Unused space is noise!!")
+
+# following 64kB is storage area 1, should be empty
+STORAGE1 = READER.read(64 * 1024)
+if all(b == 0 for b in STORAGE1):
+    print("Storage area 1 is all zero")
+elif all(b == 0xFF for b in STORAGE1):
+    print("Storage area 1 is all 0xFF")
+else:
+    print("WARNING: Storage area 1 is noise!!")
+
+# following 128 kB is bootloader
+BOOTLOADER_SECTOR = READER.read(128 * 1024)
+BOOTLOADER_HEADER = FirmwareHeader.parse(BOOTLOADER_SECTOR)
+assert BOOTLOADER_HEADER.magic == HeaderType.BOOTLOADER
+length = BOOTLOADER_HEADER.header_len + BOOTLOADER_HEADER.code_length
+BOOTLOADER = BOOTLOADER_SECTOR[:length]
+BOOTLOADER_AFTER = BOOTLOADER_SECTOR[length:]
+
+BOOTLOADER_PARSED = firmware_headers.parse_image(BOOTLOADER)
+print("Bootloader parses OK:")
+print(BOOTLOADER_PARSED.format())
+if all(b == 0 for b in BOOTLOADER_AFTER):
+    print("Bootloader padding is all zero")
+elif all(b == 0xFF for b in BOOTLOADER_AFTER):
+    print("Bootloader padding is all 0xFF")
+else:
+    print("WARNING: Bootloader padding is noise!!")
+
+# rest of the image is prodtest
+PRODTEST = READER.read()
+PRODTEST_PARSED = firmware_headers.parse_image(PRODTEST)
+print("Prodtest parses OK:")
+print(PRODTEST_PARSED.format())
+
+# save results:
+Path("boardloader.bin").write_bytes(BOARDLOADER)
+Path("bootloader.bin").write_bytes(BOOTLOADER)
+Path("prodtest.bin").write_bytes(PRODTEST)