add cortex-v7a support

How To Run:
    see TencentOS-tiny\board\ALPHA_I.MX_emmc_256ddr\README.md

TODO Next:
1. VFP support
2. fault diagnosis support
3. qemu vexpress ca9 support
4. raspberry pi support
5. SMP support

kernel/core/tos_sys.c

@@ -159,6 +159,7 @@ __API__ k_err_t tos_knl_start(void)
     k_next_task = readyqueue_highest_ready_task_get();
     k_curr_task = k_next_task;
     k_knl_state = KNL_STATE_RUNNING;
+
     cpu_sched_start();
 
     return K_ERR_NONE;