Add LTO and unused data/functions optimisations suggested by Arti

Makefile

@@ -43,13 +43,18 @@ CFLAGS =	-Wall \
 			-Werror \
 			-Wfatal-errors \
 			-Os \
+			-flto \
+			-fdata-sections \
+			-ffunction-sections \
 			-mmcu=$(BOARD) \
 			-DF_CPU=16000000UL \
 			-DGIT_DESCR=\"$(shell git describe --abbrev=6 --dirty --always --tags --long)\" \
 			-std=c11
 
 # Linker flags
-LDFLAGS = -mmcu=$(BOARD)
+LDFLAGS =	-mmcu=$(BOARD) \
+			-flto \
+			-Wl,-gc-sections
 
 OBJCOPYARGS =	-O ihex \
 				-R .eeprom