MCU + Linux + Video?
I've been doing something like this over the course of about 4 of the last 6 months (I worked on some other stuff in between). Building my own Linux board has been a tremendous learning experience, but it's also been quite challenging.

Here are some things I wish I knew before I started:


Generally, BGA packages mean you have to use fine-pitch traces and vias, and you it's difficult to assemble the boards yourself. Even if you were to succeed in soldering a BGA yourself, if you had a problem, you wouldn't know if it was due to bad soldering or bad design. The worst problem, though, is that this makes iterating on your design expensive-- around $1000 and 15 business days per rev if you live in the US.
For the love of William Shockley, start with a reference design with Android support (well, just Linux in my case). My board is based off the Atmel AT91SAM9G20 eval kit; that's saved me a great deal of headache.
Use the U-boot bootloader. It's the dominant one, which means that you can get support in more places.
Before you start laying out a board, think very carefully about how to position the components to minimize trace length. I did this to some extent, but not as much as I should have.
Buy the dev kit that is most similar to what you're planning to build, and build the bootloader and kernel from source. That will teach you a lot. If you're serious about video, I would probably try to validate whatever hardware platform I chose before laying out a board.


Good luck.