bricks/_common: use flto=auto

Newer versions of GCC complain that link time optimization is not using
parallel tasks. Adding the =auto option makes the warning go away. And
probably reduces compile time by a bit.

Author: dlech

bricks/_common/arm_none_eabi.mk

@@ -184,9 +184,9 @@ CFLAGS += -fsingle-precision-constant -Wdouble-promotion
 ifeq ($(DEBUG), 1)
 CFLAGS += -Og -ggdb
 else ifeq ($(DEBUG), 2)
-CFLAGS += -Os -DNDEBUG -flto
+CFLAGS += -Os -DNDEBUG -flto=auto
 else
-CFLAGS += -Os -DNDEBUG -flto
+CFLAGS += -Os -DNDEBUG -flto=auto
 CFLAGS += -fdata-sections -ffunction-sections
 endif