A bunch of RISC-V build fixes and some cleanups
- Add riscv64 support to grub.macros and RISC-V build fixes (davidlt)
- blscfg: Always use the root variable to search for BLS snippets
- bootstrap.conf: Force autogen.sh to use python3
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>