mirror: Ignore $(CFLAGS), use $(EXTRA_CFLAGS) in feature detection

Feature detection in the mirror repository should behave just like in
the kernel: $(CFLAGS) are ignored (overwritten), but $(EXTRA_CFLAGS) are
preserved. This is expecially important if users pass some
$(EXTRA_CFLAGS) to do a static build, for example.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

src/Makefile.feature

@@ -3,6 +3,7 @@
 pound := \#
 
 CFLAGS_BACKUP := $(CFLAGS)
+CFLAGS := $(EXTRA_CFLAGS)
 ifneq ($(LLVM),)
   CFLAGS += -Wno-unused-command-line-argument
 endif