create common.mk for os projects

Makefiles of rvos projects contains too many duplicated contents.
Cleanup and move it into a new common.mk file for os only, this will
not touch asm samples.

Finally the common..mk which was used for both asm & os is removed.

Signed-off-by: Wang Chen <wangchen20@iscas.ac.cn>

code/asm/build.mk

@@ -1,3 +1,6 @@
+# Note: The Makefile for each project is just a symbol-link to the build.mk
+#       under the "asm" folder.
+
 EXEC = test
 
 SRC = ${EXEC}.s

code/asm/rule.mk

@@ -1,4 +1,15 @@
-include ../../common.mk
+# This file will be included by the build.mk.
+
+CROSS_COMPILE = riscv64-unknown-elf-
+CFLAGS = -nostdlib -fno-builtin -march=rv32g -mabi=ilp32 -g -Wall
+
+QEMU = qemu-system-riscv32
+QFLAGS = -nographic -smp 1 -machine virt -bios none
+
+GDB = gdb-multiarch
+CC = ${CROSS_COMPILE}gcc
+OBJCOPY = ${CROSS_COMPILE}objcopy
+OBJDUMP = ${CROSS_COMPILE}objdump
 
 .DEFAULT_GOAL := all
 all: