makes build more configurable by CLI

hello-world/Makefile

@@ -2,18 +2,19 @@
 TARGET  = hello
 C_SRCS  = $(wildcard *.c) 
 HEADERS = $(wildcard *.h)
-CFLAGS += -O2 -g 
+OPT ?= -O2
+CFLAGS += $(OPT) -g 
 
 BOARD=iss
 LINK_TARGET=link
-RISCV_ARCH:=rv32i
-RISCV_ABI:=ilp32
+RISCV_ARCH:=rv32e
+RISCV_ABI:=ilp32e
 LDFLAGS += -g -Wl,--wrap=printf
 
-compiler := $(shell which riscv32-unknown-elf-gcc)
+compiler := $(shell which riscv64-unknown-elf-gcc)
 TOOL_DIR=$(dir $(compiler))
 
-TRIPLET=riscv32-unknown-elf
+TRIPLET=riscv64-unknown-elf
 BSP_BASE = ../bare-metal-bsp
 include $(BSP_BASE)/env/common-gcc.mk