riscv64 updates: change kernel image type to flat image and enable vdso (Closes: #928451)

The riscv64 architecture is changing its standard kernel image format from ELF to a flat kernel image with a PE/COFF-compatible header (similar to arm64) to make EFI stub support possible, so we need to ship arch/riscv/boot/Image instead of an ELF vmlinux. This also enables us to get rid of BBL (the RISC-V Berkeley BootLoader) and make the move to U-Boot/GRUB on riscv64.

With kernel 5.0 we can now also enable the vdso config option in the package for riscv64 as the necessary infrastructure is now in the upstream kernel (which wasn't the case when the riscv64 support was originally added to the Debian kernel package).