Another easy way to play around with Linux fit onto very modest hardware (but not 8-bit microcontrollers), which is very popular and very useful, is OpenWrt on a cheap old home WiFi router: https://en.wikipedia.org/wiki/OpenWrt
(If you want to pick up an old router for $20, WNDR3700v2, WNDR3700v4, WNDR3800, and select later ones in that family are a good starting point. The OpenWrt wiki pages for them will give specs and suggest what hardware revs are better.)
Another easy way to play around with Linux fit onto very modest hardware (but not 8-bit microcontrollers), which is very popular and very useful, is OpenWrt on a cheap old home WiFi router: https://en.wikipedia.org/wiki/OpenWrt
(If you want to pick up an old router for $20, WNDR3700v2, WNDR3700v4, WNDR3800, and select later ones in that family are a good starting point. The OpenWrt wiki pages for them will give specs and suggest what hardware revs are better.)