scripts: add support for Sercomm kernel header

scripts: add support for Sercomm kernel header

This scripts creates custom kernel header that necessary for Sercomm
mt7621 devices:
- Sercomm S3
- Beeline SmartBox Giga
- Beeline SmartBox Pro
- Beeline Smartbox Turbo
- Beeline Smartbox Turbo+
- WiFire S1500.NBN

Header format
-------------
+--------+---------------+------------------------+
| Offset | Value         | Description            |
+========+===============+========================+
| 0x0    | 53 65 72 00   | Magic "Ser."           |
+--------+---------------+------------------------+
| 0x4    | 04 00 00 01   | End offset of RootFS   |
+--------+---------------+------------------------+
|        |               | This header checksum   |
| 0x8    | d6 14 9a c1   | htonl(~crc)            |
+--------+---------------+------------------------+
| 0xc    | 02 ff ff ff   | Constant               |
+--------+---------------+------------------------+
| 0x10   | 00 01 40 00   | Kernel start offset    |
+--------+---------------+------------------------+
| 0x14   | c6 94 24 00   | Kernel length          |
+--------+---------------+------------------------+
|        |               | Kernel checksum        |
| 0x18   | e7 78 89 f1   | htonl(~crc)            |
+--------+---------------+------------------------+
| 0x1c   | 00 00 00 00   | Constant               |
+--------+---------------+------------------------+
| 0x20   | ff ff ff ff   | Constant               |
+--------+---------------+------------------------+
| 0x24   | ff ff ff ff   | Constant               |
+--------+---------------+------------------------+
| 0x28   | 00 00 00 01   | RootFS offset          |
+--------+---------------+------------------------+
|        |               | RootFS length          |
| 0x2c   | 04 00 00 00   | Always 0x4, we check   |
|        |               | UBI magic only         |
+--------+---------------+------------------------+
|        |               | RootFS checksum        |
| 0x30   | 1c fc 55 2d   | htonl(~crc)            |
|        |               | Const for UBI magic    |
+--------+---------------+------------------------+
| 0x34   | 00 00 00 00   | Constant               |
+--------+---------------+------------------------+
| 0x38   | ff ff ff ff … | Pad to 0x100           |
+--------+---------------+------------------------+

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>