A typical embedded board is based on a reference design from the manufacturer with changes to make it suitable for a particular application. It may have a temperature sensor attached via I2C, lights and buttons connected via GPIO pins, an external Ethernet MAC, a display panel via a MIPI interface, or many other things. Your job is to create a custom kernel to control all of that, so where do you start?
Some things are so simple that you can write user space code to handle them. GPIOs and simple peripherals connected via I2C or SPI are easy to control from user space, as I will explain later.
Other things need a kernel driver so you need to know how to find one and incorporate it into your build. There is no simple answer, but here are some places to look.
The most obvious place to look is the driver support page on the manufacturer's website, or you could ask them directly. In my experience, this seldom gets the result you want; hardware manufacturers are not are not particularly Linux-savvy and they often give you misleading information. They may have proprietary drivers as binary blobs or they may have source code, but for a different version of the kernel than the one you have. So, by all means try this route. I will always try to find an open source driver for the task in hand.
There may be support in your kernel already: there are many thousands of drivers in mainline Linux and there are many vendor-specific drivers in the vendor kernels. Begin by running make menuconfig (or xconfig) and search for the product name or number. If you do not find an exact match, try more generic searches, allowing for the fact that most drivers handle a range of products from the same family. Next, try searching through the code in the drivers directory (grep is you friend here). Always make sure you are running the latest kernel for your board: later kernels generally have more device drivers.
If you still don't have a driver, you can try searching online and asking in the relevant forums to see if there is a driver for a different version of Linux. If you find one, you will have to backport it to your kernel. If the kernel versions are similar, it may be easy, but if they are more than 12 to 18 months apart, the chances are that the interfaces will have changed to the extent that you will have to rewrite a chunk of the driver to integrate it with your kernel. You may want to outsource this work. If all of the above fails, you will have to find a solution yourself.