bsp,abstract-machine: add main() to call entry()

* this flow is similar with qemu-riscv-virt64
* there is still a lot of linking error

bsp/abstract-machine/Makefile

@@ -4,7 +4,7 @@ FILE_TMP = .tmp.$(FILE_MK)
 RTCONFIG_H = rtconfig.h
 
 NAME = rtthread
-SRCS = 
+SRCS = $(shell find src -name "*.c")
 CFLAGS += -DHAVE_CCONFIG_H -D__RTTHREAD__
 -include $(FILE_MK)
 include $(AM_HOME)/Makefile

bsp/abstract-machine/src/init.c

@@ -0,0 +1,5 @@
+int main() {
+  extern int entry(void);
+  entry();
+  return 0;
+}