user_features.bbclass: also process DISTRO_FEATURES_LIBC

If a feature we want to add is listed in DISTRO_FEATURES_LIBC_DEFAULT, then
add it to DISTRO_FEATURES_LIBC. Remove any features we want to remove from
both DISTRO_FEATURES_LIBC and DISTRO_FEATURES.

JIRA: SB-3931

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Merge pull request #373 from kergoth/sb-3931

user_features.bbclass: also process DISTRO_FEATURES_LIBC