clang: Enable lto when asked for via DISTRO_FEATURES
clang: Enable lto when asked for via DISTRO_FEATURES

add 'thin-lto' or 'full-lto' to DISTRO_FEATURES to enable it
instead of making it unconditional

Should fix Issue #236

Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

kraj/meta-clang,kraj/meta-clang,kraj/meta-clang