I just tried the Project Trellis open source FPGA tool chain for RISC-V. I was forced into it: a few years ago I bought the Lattice ECP5-5G Versa Dev kit (on sale at the time for $99, $200 now I think). The Lattice tools for the FPGA on this board requires a license. Well the license ran out, and I don't want to pay $900 for a Diamond license.

Anyway, I got this example from Project Trellis to work on this board. It uses this "PICO RISC-V" (the entire CPU is in a single Verilog file):

https://github.com/SymbiFlow/prjtrellis/tree/master/examples...

I think it would be straightforward to build a Linux capable RISC-V system for this board using this free tool chain. The board has SDRAM, FTDI UART and SPI-flash. It does not have an SD slot, but maybe could be wired in with the expansion headers.

This ULX3S board also looks promissing, but is not available yet:

https://www.crowdsupply.com/radiona/ulx3s

What's very cool is that low end ECP5 FPGAs are around $5. I think RISC-V on an ECP5 with LCD touch interface might be the cheapest available option for a Linux microcontroller with a touch LCD interface. By available, I mean where you can easily buy the chips on mainstream distributors (otherwise it would for sure be some AllWinner chip).