Add 'led' command

This patch allows any board implementing the coloured LED API
to control the LEDs from the console.

led [green | yellow | red | all ]  [ on | off ]

or

led [ 1 | 2 | 3 | all ]  [ on | off ]

Adds configuration item CONFIG_CMD_LED enabling the command.

Partially based on patch from Ulf Samuelsson:
http://www.mail-archive.com/u-boot@lists.denx.de/msg09593.html.

Updated based on feedback:
http://www.mail-archive.com/u-boot@lists.denx.de/msg41847.html
https://groups.google.com/d/topic/beagleboard/8Wf1HiK_QBo/discussion


* Fixed a handful of style issues.
* Significantly reduced the number of #ifdefs and redundant code
* Converted redundant code into loops test against a structure
* Made use of cmd_usage()
* Introduced a str_onoff() function, but haven't yet put it in common
* Eliminated trailing newline

Signed-off-by: Jason Kridner <jkridner@beagleboard.org>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>