GOT IT !!!
It was the f***ed-up rtc indeed! Let me explain:
I simplified my experiment from above:
- installed debian-3.8-wheezy-3.4.107 --> ok (eth0 is ok, ping heise.de works)
- survives reboot / halt and power-on / halt and dis-/reconnect power --> everything ok
Then upgrade to kernel 4.0.5 as described in your faq and then:
--> System boots till login screen (!!!) but eth0 isn't set up! No ping possible!
dmesg | grep rtc
2085-06-25 (NOT ok)
Fr 20. Mai 13:37:52 CET 1949 (NOT ok)
hwclock: The Hardware Clock registers contain values that are either invalid (e.g. 50th day of month) or beyond the range we can handle (e.g. Year 2095).
So, all date/time and clocks are wrong! Now I've set the correct date manually:
date -s "25 JUN 2015 21:00:10"
But hwclock / rtc is still going wrong:
So, I've now set hwclock to system date via:
hwclock --systohc --localtime
And voilÃ :
--> ok (!)
--> eth0 comes up !!!! ping ok!
date --> ok
hwclock --show --> ok
halt and power on
date : ok ping ok
halt power dis-reconmnect
Then I removed sdcard, copied debian-4.0-wheezy-4.1 on it, put it back in and reset
Tatataaaaaaa, it doesn't complain about rtc, it recognizes eth0 and boots normally!
Repeated that with new installation of debian 3.9-jessie-4.0.5, also works like a charm!
Hope, that this is helpful for you other guys, too!!!