led: Remove state-saving of led for toggle functionality and add toggle option to led command

* Read the led output state from GPIO instead saving state in memory when it is [re]set
* Added a toggle option to the led command

Previous discussion:
http://lists.denx.de/pipermail/u-boot/2011-May/093068.html



Changes since v1:
Fixed checkpatch errors

Signed-off-by: Joel A Fernandes <agnel.joel@gmail.com>
Signed-off-by: Jason Kridner <jkridner@beagleboard.org>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>