[coreboot] and/or busybox issue with alix2c3 serial console

[Roman Yeryomin]

I'm experimenting with coreboot (v3) on alix2c3 and still can't get serial 
console working. :( It stops right after kernel is done booting (I can see 
coreboot, filo and kernel messages just fine). I use openWRT but tried also 
another busybox based distribution (imedialinux) -- same problem. With 
original tinybios no problem. At the same time I tried voyage linux (without 
busybox) with corebooted alix and it has no such problem. It's obvious that 
this problem cannot be identified as 100% busybox's (why then it works on 
tinybios?).
While digging coreboot sources I've noticed that uart initialization is done 
twice for this board -- once in /southbridge/amd/cs5536/cs5536.c (based on 
dts config) and second time in /mainboard/pcengines/alix2c3/stage1.c.
I tried playing with these (and /southbridge/amd/cs5536/stage1.c), disabling 
one code or another, but no luck.
Tried to set MOD bits:

msr.lo = (1 << 4) | (1 << 5) | (1 << 6);
    msr.hi = 0;
    wrmsr(MDD_UART1_MOD, msr);

Also no luck.
Not enabling upper banks, however, makes uart appear in kernel as regular 
16550A (and not as NS16550):

/* Bit 1 = DEVEN (device enable)
     * Bit 4 = EN_BANKS (allow access to the upper banks)
     */
    //msr.lo = (1 << 4) | (1 << 1);
    msr.lo = 1 << 1;
    msr.hi = 0;

So... where should I go now?  I'm really stuck. Begging for help...

Regards,
Roman