firmware-utils: mktplinkfw2: fix MD5 salt

firmware-utils: mktplinkfw2: fix MD5 salt

LEDE supports few devices using TP-Link firmware format (V2 or V3):
ArcherC20i, ArcherC50, ArcherMR200, TDW8970, TDW8980, TL-WR840N v4,
TL-WR841N v13 and VR200v

Testing mktplinkfw2 tool with official (vendor generated) firmware files
for above devices has shown an error when comparing calculated and
included MD5 sum, e.g.:
> mktplinkfw2 -i Archer_C20iv1_0.9.1_3.2_up_boot\(170221\)_2017-02-21_17.14.03.bin | grep -A 1 MD5Sum1
Header MD5Sum1         : 22 5a cb 92 10 d2 95 7b df 62 9a f8 62 17 37 10 (*ERROR*)
          --> expected : ad 19 11 d1 78 98 a7 42 5f 2e 64 da 8a 34 ec cb

This problem has been verified to occur with:
Archer_C20iv1_0.9.1_3.2_up_boot(170221)_2017-02-21_17.14.03.bin
Archer MR200v1_0.9.1_1.1_up_boot_v004a.0 Build 160905 Rel.60037n.bin
TD-W8970v3_0.9.1_2.0_up_boot(160816)_2016-08-16_10.40.57.bin
TD-W8980v1_0.6.0_1.8_up_boot(150514)_2015-05-14_11.16.43.bin
Archer_VR200vv2_0.2.0_0.8.0_up_boot(161202)_2016-12-05_14.39.06.bin

For some images, e.g.:
Archer_C50v3_EU_0.9.1_0.3_up_boot[170417-rel52298].bin
TL-WR840Nv4_EU_0.9.1_4.16_up_boot[170421-rel70692].bin
TL-WR841Nv13_0.9.1_3.16_up_boot(161012).bin
mktplinkfw2 calculates zero MD5 so these has to be fixed separately:
> mktplinkfw2 -i TL-WR841Nv13_0.9.1_3.16_up_boot\(161012\).bin | grep -A 1 MD5Sum1
Header MD5Sum1         : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 (*ERROR*)
          --> expected : 6f 1d 9b 57 5d 42 14 6d bf a2 03 9d 46 7d 55 55

It's most likely that MD5 salt used in mktplinkfw2 has been always wrong
(and it's not a matter of e.g. a vendor change). Update it to fix MD5
calculation.

This has been also verified to calculate MD5 correctly for other (not
yet supported) devices, e.g.:
Archer_C3150v2_0.1.0_0.9.1_up_boot(160812)_2016-08-12_10.52.54.bin
Archer_C3200v1_0.9.1_0.1_up_boot(160704)_2016-07-04_15.48.28.bin

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Mathias Kresin <dev@kresin.me>