configs/qemu_riscv*_virt: use mainline kernel

Update the RISC-V QEMU virt machine defconfigs to use the
mainline 4.20.17 kernel instead of the version from the
riscv-linux repository.

This change has been tested with QEMU 3.1.0.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>