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-devtools/python/python3_3.12.4.bb

@@ -118,7 +118,7 @@ ${@bb.utils.contains('PACKAGECONFIG', 'tk', '', '_tkinter', d)}
 EOF
 }
 
-CPPFLAGS:append = " -I${STAGING_INCDIR}/ncursesw -I${STAGING_INCDIR}/uuid"
+CPPFLAGS += "-I${STAGING_INCDIR}/ncursesw -I${STAGING_INCDIR}/uuid"
 
 # COMPILEALL_OPTS= ensures that .pyc are not compiled in parallel
 # This was found to lock up builds, break reproducibility, and produce strange file ownership