1. Add poll/select APIs;
2. Add wait queue implementation;
3. Remove DFS_STATUS_* code and use errno;
4. Add pipe API;
5. Use a standalone fops in DFS;

1. Add POSIX termios implementation;
2. Add POSIX signals implementation;
3. Add stdio for each libc.

1. Add signal implementation;
2. Change the 'rt_uint8_t' of cmd to 'int';

1. Remove components.h file;
2. Add libc_* files for standard libc definitions;
3. Add rtdbg.h file for simple debug log;
4. Add single list implementation;
5. Change the 'rt_uint8_t' type of cmd to 'int'.

[Finsh] Stop push new history when current cmd is same as last cmd.

Use uint8_t/uint32_t etc to replace u_char/u_long etc;
Use getchar for shell input.

The master branch will be frozen util v3.0.0 updated.

[lwIP] Fix the lwIP 2.0.2 IAR compile error

[BSP] Add SD driver in stm32f429-apollo bsp. (TODO: 32G SD card not work yet)

[BSP] Add menuconfig for stm32f429-apollo bsp.