libcgroup: fix passing CFLAGS/LDFLAGS to configure

LIBCGROUP_CONF_ENV is a string used to pass environment variables to the
confgiure script, '+=' operator doesn't have any special meaning inside
it, so CFLAGS+=... is passed to shell, overwriting previous CFLAGS
value. Replace CFLAGS+="..." with CFLAGS="$(TARGET_CFLAGS) ...".

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>