I am working on my own quadcopter and I would like to stream video from it to my computer. I know how to do it using Raspberry Pi and wifi dongle hooked to its usb port, but I don't want that solution because it's bulky and heavy. I need something like what ARDRone did with their ARDrone2.0 quadcopter ARDrone 2.0. They have an onboard microprocessor, 512MB RAM memory, tiny HD camera and a wifi module. All of it fits in a very compact space that is considerably smaller than that needed by Raspberry Pi. I've been searching online a lot trying to find out how to build these systems, but I couldn't find anything. Where do I need to start to learn how to build these custom systems without using Raspberry Pi, Arduinos, etc. Thanks!
tweaking off-the-shelf hardware to reduce mass
In practice, people often find that they can get most of the advantages of full-custom systems with much less time and money by tweaking off-the shelf PCBs.
Even if I were convinced I needed to make a full-custom system, my first prototype would use off-the-shelf hardware just to make sure the system as a whole "works" and I haven't forgotten some critical subsystem, even if the prototype weighs 10 times as much as I expect the full-custom version to weigh.
Most of the mass of a typical computer is in the enclosure and the connectors.
My second prototype would use mostly off-the-shelf hardware, tearing off the enclosures I don't need; desoldering the connectors and other heavy components for stuff I don't use; and desoldering heavy connectors and replacing them with lighter connectors or direct wire connection.
I hear "The Ben Heck Show" frequently demonstrates a similar process. The "WiFi Booster Mod by DBS" has some pretty pictures illustrating desoldering heavy connectors and replacing them with direct point-to-point soldered wires.
Full-custom Linux PCBs
In principle you could minimize the size and mass of the electronics by designing your own full-custom PCB(s).
If I were convinced I needed to make such a full-custom system, I would start from some open-hardware motherboards that run Linux and tweak the design slightly for my third prototype.
Wow, someone complaining that the Raspberry Pi is "bulky and heavy" ? Do you know nothing about the Cray-1 or the Cray X-MP that several people -- "Help Chris boot his Cray-1 supercomputer", "Raspberry Pi versus Cray X-MP supercomputer", "A Cray for $35", etc. -- claim can be replaced by a Raspberry Pi?