values for cnt_os_version.unknown_subrevision found

[uedvt359]

I've been analyzing NPKs for pyNetinstall and I believe I figured out the unknown_subrevision field in the version number: it signifies alpha, beta, rc and final versions - it's simply the lower case ASCII letter 'a', 'b', 'c' or 'f'.

i've ran this against a bunch of npks i had lying around:

for v in ("7.18.1", "7.19rc2-arm64", "7.20beta2-arm64", "7.15alpha218-arm64"):
    with open(f"routeros-{v}.npk", "rb") as f:
        x = f.read(100)

    print("routeros-{v}.npk")
    print("version", struct.unpack("B1sBB",x[0x24:0x28]))

routeros-7.18.1.npk
version (1, b'f', 18, 7)
routeros-7.19rc2-arm64.npk
version (2, b'c', 19, 7)
routeros-7.20beta2-arm64.npk
version (2, b'b', 20, 7)
routeros-7.15alpha218-arm64.npk
version (218, b'a', 15, 7)