base: make feature backfilling happen earlier

base: make feature backfilling happen earlier

Backfilling DISTRO_FEATURES and MACHINE_FEATURES with _append statements happens
too late to use those variables with conditional inherits, like this:

inherit ${@base_contains('DISTRO_FEATURES','sysvinit','update-rc.d_real','',d)}

Instead, do the backfilling at ConfigParse time so that it happens earlier in
the parse, which results in that inherit behaving as expected when sysvinit was
backfilled.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>