This patch does the following:
- Update the current linux-mainline_5.1 recipe to AUTOREV from the
stable branch
- Use the linux-mainline_5.1 by default for the HiFive Unleashed
- Remove the older linux-riscv_5.0 recipe
- Rename the common include to linux-mainline-common.inc
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>