Closes#198#200
Making it a soft panic makes it more involved with a bit of code duplication - setting up mgmt requires setting up the interface and sockets. Maybe can be done a bit cleaner.
```
[spaqin@hera:~/m-labs/artiq-zynq]$ artiq_sinara_tester
****** Sinara system tester ******
[...]
ConnectionRefusedError: [Errno 111] Connection refused
[spaqin@hera:~/m-labs/artiq-zynq]$ artiq_coremgmt -D 192.168.1.56 log
[ 0.000067s] INFO(runtime): NAR3/Zynq7000 starting...
[ 0.005238s] INFO(runtime): detected gateware: GenericMaster
[ 0.016152s] INFO(libboard_zynq::i2c): PCA9548 detected
[ 0.023004s] WARN(runtime): config initialization failed: SD error: Card initialization error: No card inserted, check if the card is inserted properly.
[ 0.036730s] WARN(runtime::rtio_clocking): error reading configuration. Falling back to default.
[ 0.213000s] ERROR(runtime::rtio_clocking): RTIO PLL failed to lock
[ 0.224443s] INFO(libboard_zynq::i2c): PCA9548 detected
[ 0.256197s] INFO(runtime::comms): network addresses: MAC=e8-eb-1b-13-49-8b IPv4=192.168.1.56 IPv6-LL=fe80::eaeb:1bff:fe13:498b IPv6: no configured address
[ 0.270183s] ERROR(runtime::comms): There has been an error configuring the device: RTIO PLL failed to lock. Only mgmt interface will be available.
[ 4.000095s] INFO(libboard_zynq::eth): eth: got Link { speed: S1000, duplex: Full }
[ 33.148521s] INFO(runtime::mgmt): received connection
```
Reviewed-on: M-Labs/artiq-zynq#199
Co-authored-by: mwojcik <mw@m-labs.hk>
Co-committed-by: mwojcik <mw@m-labs.hk>
