How to create a defconfig file from a .config?

For your platform, in a new defconfig file, yes. In fact this is the safest way to create a new defconfig. If you manually remove config entries from an existing config file to create a new one, you are likely to get dependency issues and during build, it might restart the kernel config and give you prompts for selecting individual config options.

I think you have to do just one command and use the created file as you want to.

% make savedefconfig 
% cp defconfig arch/arm/configs/my_cool_defconfig

(Pay attention to the filename template that is used for defconfig)

To get all possible targets just run

% make help

As noted by Adam Miller followed by Jeremy, users of Buildroot distribution can use wrappers for that purpose, i.e. (per Buildroot manual, section 8.1):

• linux-savedefconfig for linux
• barebox-savedefconfig for barebox bootloader
• uboot-savedefconfig for U-Boot bootloader

make savedefconfig minimizes the generated defconfig skipping redundant configs that are implied by others.