Clean Up SBI launch

In this release we make use of SBI a0 register to identify the newly launched hart (significant clean up of main and entry_sbi.S), got the Banana Pi F3 Uart to work (simplifies configuration since all boards can enable this now) and did another README documentation pass.

Add support for BPI-F3 board

Add partial support for Banana Pi BPI-F3 board and clean up some README documentation.

Add support for VisionFive 2 board

Add more formal support for VF2 board using UART0_direct option that side steps the SBI issue observed with more recent board firmware.

Initial unmatched ramdisk version

This version adapts xv6 to run on Sifive Unmatched rev B. See Errata for known issues.
v0.1.zip