Fix up Arm builds with nix-shell. (#8312)

shell.nix

@@ -45,6 +45,6 @@ stdenv.mkDerivation {
     ++ lib.optional arm [ gcc-arm-embedded ]
     ++ lib.optional teensy [ teensy-loader-cli ];
 
-  CFLAGS = lib.optional avr avr_incflags;
-  ASFLAGS = lib.optional avr avr_incflags;
+  AVR_CFLAGS = lib.optional avr avr_incflags;
+  AVR_ASFLAGS = lib.optional avr avr_incflags;
 }

tmk_core/avr.mk

@@ -21,7 +21,9 @@ COMPILEFLAGS += -fdata-sections
 COMPILEFLAGS += -fpack-struct
 COMPILEFLAGS += -fshort-enums
 
-CFLAGS += $(COMPILEFLAGS)
+ASFLAGS += $(AVR_ASFLAGS)
+
+CFLAGS += $(COMPILEFLAGS) $(AVR_CFLAGS)
 CFLAGS += -fno-inline-small-functions
 CFLAGS += -fno-strict-aliasing