native/nativesdk: Stop overriding unprefixed *FLAGS variables

We're currently encouraging an "arms race" with the FLAGS variables since
a recipe might want to set a specific flag for all variants but to do so,
+= won't work due to the assignment in the native/nativesdk class files. This
means recipes are using append.

Since the default variables are constructed out of TARGET_XXX variables and
we redefine these, there is no need to re-define the un-prefixed variables. If
we drop that, the += appends and similar work and recipes don't have to resort
to append.

Change the classes and cleanup a number of recipes to adapt to the change. This
change will result in some flags appearing to some native/nativesdk variants
but that is probably what was originally expected anyway.

(From OE-Core rev: a157b2f9d93428ca21265cc860a3b58b3698b3aa)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-connectivity/bind/bind_9.18.27.bb

@@ -44,7 +44,7 @@ EXTRA_OECONF = " --disable-auto-validation \
                  --sysconfdir=${sysconfdir}/bind \
                  --with-openssl=${STAGING_DIR_HOST}${prefix} \
                "
-LDFLAGS:append = " -lz"
+LDFLAGS += "-lz"
 
 # dhcp needs .la so keep them
 REMOVE_LIBTOOL_LA = "0"