This commit adds a tool to send out, through ethernet, a complete log of
DAC values for the main oscillator. It allows to see if there are
oscillations (and there currently are) and what happens when tracking
is lost and recovered.

To activate the feature

  * add CONFIG_DAC_LOG (depends on CONFIG_DEVELOPER)

  * activate on the console: "daclog <ipaddr> <macaddr>"

  * collect data on your target host

     netcat -u -l -p 1050  | od -An -t u2 -v -w2 --endian=big > daclog.txt

  * stop it when done, on the console: "daclog off"
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>