[vmm] add realview-pb-a8 VMM support

[msh] don't complain on empty command

Just branch out and no complain on command length == 0;

Msh next

When DFS_USING_WORKDIR is enabled, msh will try to chdir if the command
is not found. For example, if there is folder named "SD", and there is
no `SD` command and `SD.mo` module, typing "SD" in msh will cd to the
folder.

In more detail, the execution order right now is:

1. built-in command
2. module
3. cd to path

If you have a folder named "ps", you have to run `cd ps` to change the
working dir.

Msh can execute module directly. So try to auto-complete the path when
user press TAB. There is a chance that the path is a module.

modify the pixel format to RGB565 in sdl_fb.c

2.adjust some viable and micro name
3.some other small fix

Revert "Merge pull request #241 from bright-pan/master"

This reverts commit 5aeedc4f, reversing
changes made to 3d80e414.

This merge breaks every uart driver in all BSPs. @bright-pan , is there
any way to get this done without breaking existing code?

Fix rt_serial_control() bug for customized command and indent the code

Add RTS/CTS option in the serial framework. 

Add uart4 and uart5 device driver.

Add hardware flow support.
Use new struct serial_ringbuffer.

add JavaScript Interpreter

Add supports for UART0 to UART5 of beaglebone(black)