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-core/systemd/systemd-boot_255.6.bb

@@ -8,10 +8,10 @@ DEPENDS = "intltool-native libcap util-linux gperf-native python3-jinja2-native
 inherit meson pkgconfig gettext
 inherit deploy
 
-LDFLAGS:prepend = "${@ " ".join(d.getVar('LD').split()[1:])} "
+LDFLAGS =+ "${@ " ".join(d.getVar('LD').split()[1:])} "
 
 EFI_LD = "bfd"
-LDFLAGS:append = " -fuse-ld=${EFI_LD}"
+LDFLAGS += "-fuse-ld=${EFI_LD}"
 
 do_write_config[vardeps] += "EFI_LD"
 do_write_config:append() {