Build Commands LE 10.0.x for the R40?

Due to historical reasons, DEVICE means SoC now, so DEVICE=R40. If you want to make image instead of just update tar file, you should also add UBOOT_SYSTEM=bananapi-m2u.

Note that Bananapi M2U is not really a good choice for media device. R40 doesn't support HEVC, doesn't have power management coprocessor, which means there is no stand-by implemented, and supports only HDMI 1.4. If you want relatively cheap, solid TV box, Tanix TX6 (Allwinner H6) or clones are good choice, at least when looking at Allwinner based devices.