Rasbperry Pi is a popular choice as a SoC / SBC Linux board. But you have to use their custom linux kernel. Are there Linux boards with decent mainline Linux kernel support?
- Just google x86 SBCs and skip ARM. There are a bunch of options using AMD and Intel SoCs such as ODROID-H3. 
- You most likely do not want to run a mainline kernel / system. Run Armbian instead, it is Debian optimized for SBCs, it has a very good track record and sometimes is the only option after manufacturers stop creating images for their old boards. - Generic images / mainline kernel might underperform in your board, the GPIO and other low level components will, most likely, not work and you might burn your storage as logging and other I/O intensive operations aren’t tweaked for SD/eMMC. Armbian aims to fix all those issues and provides continuous system and kernel updates long after the manufacturer stops doing so. 
- Armbian or DietPi. - Armbian or DietPi. - OP asked for hardware, not a new distribution. 
 
- my info could be out of date on this, but the last time i looked into it, the khadas vim3 was the most powerful arm sbc with mainline linux support. 




