ramips: parameterize some values in Build/znet-header

ramips: parameterize some values in Build/znet-header

Parameterize magic number and header length to use the device-specific
values.

example:

On WRC-X1800GS, an additional ELECOM-specific header and a magic number
"COMC" are required.

Stock FW v1.18:

$ hexdump -n $((0x40080)) -C wrc-x1800gs_v1.18.bin
00000000  45 4c 45 43 4f 4d 00 00  57 52 43 2d 58 31 38 30  |ELECOM..WRC-X180|
00000010  30 47 53 00 00 00 00 00  00 00 00 00 00 00 00 00  |0GS.............|
00000020  00 00 00 00 00 00 00 00  31 2e 31 38 00 00 00 00  |........1.18....|
00000030  00 00 00 00 00 00 00 00  43 4f 4d 43 04 00 ac 00  |........COMC....|
00000040  40 f0 49 74 b1 e8 6a ca  e4 20 65 1f 34 2e 30 34  |@.It..j.. e.4.04|
00000050  28 58 56 46 2e 31 29 62  31 37 00 00 00 00 00 00  |(XVF.1)b17......|
00000060  00 00 00 00 00 00 00 00  ff ff ff ff ff ff ff ff  |................|
00000070  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |................|
*
00040000  d0 0d fe ed 00 36 9c c0  00 00 00 38 00 36 9a e0  |.....6.....8.6..|
00040010  00 00 00 28 00 00 00 11  00 00 00 10 00 00 00 00  |...(............|
00040020  00 00 00 6c 00 36 9a a8  00 00 00 00 00 00 00 00  |...l.6..........|
00040030  00 00 00 00 00 00 00 00  00 00 00 01 00 00 00 00  |................|
00040040  00 00 00 03 00 00 00 04  00 00 00 62 65 54 97 8f  |...........beT..|
00040050  00 00 00 03 00 00 00 28  00 00 00 00 4d 49 50 53  |.......(....MIPS|
00040060  20 4f 70 65 6e 57 72 74  20 46 49 54 20 28 46 6c  | OpenWrt FIT (Fl|
00040070  61 74 74 65 6e 65 64 20  49 6d 61 67 65 20 54 72  |attened Image Tr|
00040080

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>