This means BORING_SSL_PRECOMPILED_BCM_O is now
BORING_BSSL_PRECOMPILED_BCM_O.
Prefix BORING_BSSL_ has been chosen because that's the
one that is used the most among all the variables
the build script uses.
Using a struct improves navigation of the build script,
as we can rely on rust-analyzer to help us check how
a feature flag or an environment variable is used,
as opposed to grepping for multiple env::var calls
or #[cfg] attributes.
This commit also removes some obsolete blocks of code
related to the now defunct ndk-old-gcc and fuzzing features.
Anthony Ramine