Switch to the linux-rpi4-mainline kernel, which is the 5.4 branch from Raspberry Pi, instead of using pure mainline.
Some stuff, like v3d, still does not work right on mainline, so using the rpi tree directly should enable that.

Signed-off-by: Dan Johansen <strit@manjaro.org>

[oc2][on2][pbpro][rock64][rpi3][rpi4][vim1][vim2] add bootsplash packages. Only enabled on select devices