I have connected to the serial port on the iMx6 board, you will have to solder wires to the pins labelled "debug", that will not be all though.
Normally (is there any such thing?) when doing kernel work it is a good idea to have serial
console access, while this is possible with the Solo CPU, it is not convenient. There is a
console port which is labelled DEBUG on the CPU board. It is disabled in u-boot, and it shares
pins with the ADV7610 HDMI decoder. This can be disabled in the solo.dts file and a new solo.dtb
created then the serial can be enabled in the kernel. That is what I did.
Lots of work for only a bit of gain. Only necessary if you have a kernel that won't boot,
and don't mind digging into the Solo and soldering wires to it.
Or cutting a hole in the side of the Solo and chipping away the glue that secures the SD.
I bought a crashed Solo for just this purpose and it was well worth it.
If you really need serial access it is invaluable, if you don't, it is a royal PITA.
BTW, the stock u-boot can't be interrupted, you have to recompile u-boot with a few changes to the source and default configuration, but as u-boot works ok with the stock kernel, it's not really necessary unless you want to get kernel 3.14.x working as you need a newer version of u-boot for that.