The spi flash chip is MX25U12832F 128Mbit spi flash from MXIC(旺宏电子). The package of this chip is WSON-8.
Since it has only 16 MByte of space, it can only store the uboot binary.
You can write the binary to the spi flash via the RK flash tool, or use the dd command to write to the spi flash on a linux system running on the board.
There is no PXE support on 5b right now. If you are going to use PXE, you should write a PXE capable uboot to spi flash.
You can follow the “How to use serial console” guide form wiki to read logs and control it.
M2 E-key solt only support PCIE2.0 on 1 lane.
I recommend using an external SOC heatsink. If there is no heatsink, the SOC will automatically underclock to ensure that it will not burn out. At this point, the CPU portion of the SOC runs at a frequency lower than 1.4GHz. The above data is tested at an ambient temperature of 30 degrees Celsius.
The speed of M2 M-key you can follow this link to see the speed I tested with PM9a1.