Confused, Is raspberry pi a real device for learning embedded linux or just for hobby?
The Raspberry Pi is an embedded Linux system.  It is running on an ARM and will give you some of the ideas of embedded design.  Whether it is "embedded enough" is a question of how far you want to go.

There are effectively two halves of embedded Linux programming.  In many situations, the developer will wear both hats, but the required knowledge can be slightly different.


Systems programming.  This is the part about getting a bootloader working, the kernel building, and driver development.
Application programming.  This is writing the actual application that the system is designed for.  This uses the system APIs and userland programming techniques.


The Raspberry Pi is especially well suited for learning the application programming role.  All of the booting and kernel building stuff is done for you, and you can get right to what you need to do.  Some people will deride it as working on a mini-PC, but that is intentional.  It is made to be easy to work with.

On the other hand, Raspberry Pi can also be a platform for learning the systems development side.  This requires a decent amount of hardware knowledge, the ability to read datasheets, and a good understanding of both the processor and Linux boot process.  This is a lot of stuff to learn! But you can take it in pieces since you have a working example.  To learn this stuff, you really need to understand what the developers of the platform have done.  To really get it, you have to do parts of it yourself.  It seems like reinventing the wheel, and it is, but it is you doing the reinventing, and you will learn things in the process. 

As an aside, I find some professional developers to be very snobby about using "hobby" boards to learn.  I've done professional Linux systems development, and I'm going to tell you it's silly to think that way. We live in a time with amazing access to learning resources.  Learn where you can.