boards/native: add deprecation warnings for make all-debug

boards/native/Makefile.include

@@ -128,6 +128,7 @@ endif
 all: # do not override first target
 
 all-debug: all
+	@$(COLOR_ECHO) '$(COLOR_RED)`all-debug` is deprecated, just use `all`$(COLOR_RESET)'
 
 all-gprof: all
 

boards/native/doc.txt

@@ -20,4 +20,12 @@ OS/RIOT/images/Native.jpg)
 - PWM: Dummy PWM
 - QDEC: Emulated according to PWM
 - SPI: Runtime configurable - `/dev/spidev*` are supported (Linux host only)
+
+# Building native with debug flags
+@deprecated `make all-debug` is deprecated; `make all` now builds with debugs
+            flags by default. The target will be removed after the 2020.10
+            release.
+
+To build with debug flags for `native` use `make all-debug` instead of
+`make all`
  */