toolchains selection issue when building BBL about linux-on-litex-rocket

[gsomlo]

In my experience, riscv64-unknown-elf is a subset of riscv64-unknown-linux-gnu. If the former works for you but the latter does not, it's probably because the latter is somehow misconfigured or missing part of its installation. The prebuilt toolchain linked in the README (at http://www.contrib.andrew.cmu.edu/~somlo/BTCP/RISCV-toolchain.tar.xz) has both *-elf and *-linux-gnu, and both work just fine for me. Of course, *-elf is sufficient to build self-contained blobs (e.g., firmware, kernel), and *-linux-gnu is required to build programs linked against glibc, so definitely feel free to restrict yourself to using *-elf whenever that's suitable!

Hope this helps!